capf, derek ,and TNO: Guessing from previous bouts of this issue on TViXes, I think that the recordings actually don't have valid audio PIDs. It would be instructive to see a short soundless recording from capf's TViX to confirm that.
It used to be fairly common for U.S. DTV stations to mess up DD5.1 audio occasionally. But broadcasters very seldom make that mistake these days so I'm thinking that this is more likely a signal strength issue or a firmware bug interacting with some irregularity in capf's NBC station's transport stream. In the latter case, DViCo probably needs to see a clip of the complete transport stream rather than a TViX recording to fix it.
A signal strength issue, however, would be likely to affect recordings during certain times of the day or in certain seasons or weather conditions. In this case, the episodes of lacking sound might tend to be at night or correlated to some other external factor, which may be very hard to pin down unless one keeps a close eye on the "signal strength" indicator.