Add me to the list of affected 40XV645U users.
I have a HTPC connected to HDMI 1, and after an hour or so of viewing, the audio gets garbled over the course of a few seconds. I have been fixing it by cycling through the inputs, which works, but is annoying to have to do.
At this point, I would think that this is a Toshiba issue, seeing as how we've got a handful complaining of the same issue with different sources...
Thought I'd go ahead and add my firmware date and version:
May 18, 2009
Build Date: June 2009