This just started happening last week with a certain channel.... I switch to the channel in livetv, it gets a lock, and then just sits there.... the audio/video never appears. Here's the output from mythfrontend:

2007-08-26 21:31:36.350 RingBuf(/var/lib/mythtv//1031_20070826213134.mpg): OpenFile(myth://192.168.0.195:6543/1031_20070826213136.mpg, 12)
2007-08-26 21:31:39.668 RingBuf(/var/lib/mythtv//1031_20070826213136.mpg): CalcReadAheadThresh(4000 KB)
-> threshhold(146 KB) min read(32 KB) blk size(64 KB)
2007-08-26 21:31:39.746 NVP: Waiting for prebuffer.. 0 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:39.995 NVP: Waiting for prebuffer.. 1 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:40.244 NVP: Waiting for prebuffer.. 2 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:40.509 NVP: Waiting for prebuffer.. 3 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:40.672 RingBuf(/var/lib/mythtv//1031_20070826213136.mpg): Taking too long to be allowed to read..
2007-08-26 21:31:40.774 NVP: Waiting for prebuffer.. 4 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:41.039 NVP: Waiting for prebuffer.. 5 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:41.288 NVP: Waiting for prebuffer.. 6 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:41.553 NVP: Waiting for prebuffer.. 7 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:41.819 NVP: Waiting for prebuffer.. 8 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:42.016 RingBuf(/var/lib/mythtv//1031_20070826213136.mpg): Waited 1.0 seconds for data to become available...
2007-08-26 21:31:42.016 Checking to see if there's a new livetv program to switch to..
2007-08-26 21:31:42.067 NVP: Waiting for prebuffer.. 9 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:42.232 NVP: Prebuffer wait timed out 10 times.
2007-08-26 21:31:42.316 NVP: Waiting for prebuffer.. 0 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:42.548 NVP: Waiting for prebuffer.. 1 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:42.813 NVP: Waiting for prebuffer.. 2 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.079 NVP: Waiting for prebuffer.. 3 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.377 NVP: Waiting for prebuffer.. 4 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.384 RingBuf(/var/lib/mythtv//1031_20070826213136.mpg): Waited 1.0 seconds for data to become available...
2007-08-26 21:31:43.384 Checking to see if there's a new livetv program to switch to..
2007-08-26 21:31:43.513 AFD: Stream #0, has id 0x49 codec id MPEG2VIDEO, type Video, bitrate 8000000 at 0x0x2aaaace21e60
2007-08-26 21:31:43.528 VideoOutputXv: InputChanged(720,576,1.77778)
2007-08-26 21:31:43.529 VideoOutputXv: DiscardFrames(1)
2007-08-26 21:31:43.529 VideoBuffers::DiscardFrames(1): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.529 VideoBuffers::DiscardFrames(): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done()
2007-08-26 21:31:43.529 VideoBuffers::DiscardFrames(1): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done
2007-08-26 21:31:43.529 VideoOutputXv: DiscardFrames() 3: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done()
2007-08-26 21:31:43.529 VideoOutputXv: DiscardFrames(1)
2007-08-26 21:31:43.530 VideoBuffers::DiscardFrames(1): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.530 VideoBuffers::DiscardFrames(): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done()
2007-08-26 21:31:43.530 VideoBuffers::DiscardFrames(1): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done
2007-08-26 21:31:43.530 VideoOutputXv: DiscardFrames() 3: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done()
2007-08-26 21:31:43.533 VideoOutputXv: Closing XVideo port 274
2007-08-26 21:31:43.539 VideoOutputXv: XvMCTex: Init failed
2007-08-26 21:31:43.539 VideoOutputXv: @ j=3 Looking for flag[s]: XvInputMask XvImageMask
2007-08-26 21:31:43.540 VideoOutputXv: Adaptor#0: NV17 Video Texture has flag[s]: XvInputMask XvImageMask
2007-08-26 21:31:43.540 VideoOutputXv: Grabbed xv port 274
2007-08-26 21:31:43.540 VideoOutputXv: XVideo surface found on port 274
2007-08-26 21:31:43.540 VideoOutputXv: XVideo Adaptor Name: 'NV17 Video Texture'
2007-08-26 21:31:43.540 VideoOutputXv: XVideo Format #0 is 'YUY2'
2007-08-26 21:31:43.540 VideoOutputXv: XVideo Format #1 is 'YV12'
2007-08-26 21:31:43.540 VideoOutputXv: XVideo Format #2 is 'UYVY'
2007-08-26 21:31:43.541 VideoOutputXv: XVideo Format #3 is 'I420'
2007-08-26 21:31:43.541 VideoOutputXv: Using XVideo Format 'I420'
2007-08-26 21:31:43.541 VideoOutputXv: CreateShmImages(32): video_dim: 720x576
2007-08-26 21:31:43.592 VideoOutputXv: Chromakeying not possible with this XVideo port.
2007-08-26 21:31:43.592 Display Rect left: 0, top: 75, width: 800, height: 450, aspect: 1.33333
2007-08-26 21:31:43.592 Video Rect left: 14, top: 12, width: 691, height: 553, aspect: 1.77778
2007-08-26 21:31:43.921 NVP: ClearAfterSeek(1)
2007-08-26 21:31:43.922 VideoOutputXv: ClearAfterSeek()
2007-08-26 21:31:43.922 VideoOutputXv: DiscardFrames(0)
2007-08-26 21:31:43.922 VideoBuffers::DiscardFrames(0): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.922 VideoBuffers::DiscardFrames(0): AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done
2007-08-26 21:31:43.922 VideoOutputXv: DiscardFrames() 3: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA -- done()
2007-08-26 21:31:43.923 detectInterlace(Detect Scan, Interlaced Scan, 25, 576) ->Interlaced Scan
2007-08-26 21:31:43.923 AFD: EIA-708 caption service #1 is in the English language.
2007-08-26 21:31:43.924 AFD: Looking for decoder for MPEG2VIDEO
2007-08-26 21:31:43.924 AFD: Opened codec 0x2aaaace2e020, id(MPEG2VIDEO) type(Video)
2007-08-26 21:31:43.924 AFD: Stream #1, has id 0x52 codec id AC3, type Audio, bitrate 0 at 0x0x2aaaace28f40
2007-08-26 21:31:43.924 AFD: Looking for decoder for AC3
2007-08-26 21:31:43.927 AFD: Opened codec 0x2aaaace2e3f0, id(AC3) type(Audio)
2007-08-26 21:31:43.927 AFD: Audio Track #1 is A/V stream #1 and has 0 channels in the English language(6647399).
2007-08-26 21:31:43.960 RingBuf(/var/lib/mythtv//1031_20070826213136.mpg): CalcReadAheadThresh(8000 KB)
-> threshhold(292 KB) min read(32 KB) blk size(128 KB)
2007-08-26 21:31:43.961 AFD: Selected track 1: English AC3 ?ch (A/V Stream #1)
2007-08-26 21:31:43.961 AFD: Initializing audio parms from audio track #1
2007-08-26 21:31:43.961 AFD: Audio format changed
from id(NONE) -1Hz -1ch 0bps ; id(NONE) -1Hz -1ch 0bps
to id( AC3) 0Hz 0ch 0bps pt ; id( AC3) 48000Hz 2ch 16bps pt
2007-08-26 21:31:43.966 AO: Killing AudioOutputDSP
2007-08-26 21:31:43.966 Opening OSS audio device '/dev/dsp'.
2007-08-26 21:31:43.967 AO: Audio fragment size: 4096
2007-08-26 21:31:43.968 AO: Audio Stretch Factor: 1
2007-08-26 21:31:43.969 AO: Ending reconfigure
2007-08-26 21:31:43.969 NVP: Enabling Audio
2007-08-26 21:31:43.969 Dec: Trying to select track (w/lang)
2007-08-26 21:31:43.969 Dec: Selecting first track
2007-08-26 21:31:43.969 Dec: Selected track #1 in the Unknown language(0)
2007-08-26 21:31:43.969 Dec: Selected track #1 in the English language(6647399)
2007-08-26 21:31:43.969 Resyncing position map. posmapStarted = 0 livetv(1) watchingRec(0)
2007-08-26 21:31:43.970 AO: kickoffOutputAudioLoop: pid = 5251
2007-08-26 21:31:43.971 Position map filled from DB to: 60
2007-08-26 21:31:43.971 SyncPositionMap watchingrecording, from DB: 6 entries
2007-08-26 21:31:43.971 write -> 18 39 QUERY_RECORDER 7[]:[]GET_FRAMES_WRITTEN
2007-08-26 21:31:43.971 AO: OutputAudioLoop: Play Event
2007-08-26 21:31:43.972 AO: Broadcasting free space avail
2007-08-26 21:31:43.972 read <- 18 8 0[]:[]98
2007-08-26 21:31:43.972 Filling position map from 61 to 98
2007-08-26 21:31:43.972 write -> 18 52 QUERY_RECORDER 7[]:[]FILL_POSITION_MAP[]:[]61[]:[]98
2007-08-26 21:31:43.973 read <- 18 83 0[]:[]72[]:[]0[]:[]76892[]:[]0[]:[]84[]:[]0[]:[]90240[]:[]0[]:[]9...
2007-08-26 21:31:43.973 Position map filled from Encoder to: 96
2007-08-26 21:31:43.973 SyncPositionMap watchingrecording total: 9 entries
2007-08-26 21:31:43.973 SyncPositionMap, new totframes: 96, new length: 3, posMap size: 9
2007-08-26 21:31:43.974 AFD: Partial position map found
2007-08-26 21:31:43.974 AFD: Successfully opened decoder for file: "/var/lib/mythtv//1031_20070826213136.mpg". novideo(0)
2007-08-26 21:31:43.974 NVP: Waiting for prebuffer.. 5 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2007-08-26 21:31:43.977 NVP: DoPlay: rate: 25 speed: 1 skip: 1 => new interval 40000
2007-08-26 21:31:43.978 Set video sync frame interval to 40000
2007-08-26 21:31:43.978 NVP: Stretch Factor 1, allow passthru
2007-08-26 21:31:43.978 Resyncing position map. posmapStarted = 0 livetv(1) watchingRec(1)
2007-08-26 21:31:43.979 Position map filled from DB to: 60
2007-08-26 21:31:43.979 SyncPositionMap watchingrecording, from DB: 6 entries
2007-08-26 21:31:43.979 write -> 18 39 QUERY_RECORDER 7[]:[]GET_FRAMES_WRITTEN
2007-08-26 21:31:43.980 read <- 18 8 0[]:[]99
2007-08-26 21:31:43.980 Filling position map from 61 to 99
2007-08-26 21:31:43.980 write -> 18 52 QUERY_RECORDER 7[]:[]FILL_POSITION_MAP[]:[]61[]:[]99
2007-08-26 21:31:43.981 read <- 18 83 0[]:[]72[]:[]0[]:[]76892[]:[]0[]:[]84[]:[]0[]:[]90240[]:[]0[]:[]9...
2007-08-26 21:31:43.981 AO: OutputAudioLoop: audio paused
2007-08-26 21:31:43.981 Position map filled from Encoder to: 96
2007-08-26 21:31:43.981 AO: 65536 bytes free on soundcard
2007-08-26 21:31:43.981 SyncPositionMap watchingrecording total: 9 entries
2007-08-26 21:31:44.021 AO: 63100 bytes free on soundcard
2007-08-26 21:31:44.029 AO: 60440 bytes free on soundcard
2007-08-26 21:31:44.036 AO: 61932 bytes free on soundcard
2007-08-26 21:31:44.045 AO: 59408 bytes free on soundcard
2007-08-26 21:31:44.052 AO: 60908 bytes free on soundcard
2007-08-26 21:31:44.060 AO: 62468 bytes free on soundcard
2007-08-26 21:31:44.068 AO: 59884 bytes free on soundcard
2007-08-26 21:31:44.076 AO: 61440 bytes free on soundcard
2007-08-26 21:31:44.084 AO: 58856 bytes free on soundcard
2007-08-26 21:31:44.092 AO: 60416 bytes free on soundcard
2007-08-26 21:31:44.100 AO: 61932 bytes free on soundcard
2007-08-26 21:31:44.108 AO: 59384 bytes free on soundcard
2007-08-26 21:31:44.116 AO: 60908 bytes free on soundcard
2007-08-26 21:31:44.124 AO: 62456 bytes free on soundcard
2007-08-26 21:31:44.132 AO: 59884 bytes free on soundcard
2007-08-26 21:31:44.141 AO: 61468 bytes free on soundcard
2007-08-26 21:31:44.148 AO: 58872 bytes free on soundcard
2007-08-26 21:31:44.156 NVP: Waiting for prebuffer.. 6 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Well I turned to that channel tonight and now it seems to be working again.... weird.