I have often seen a problem where I have the same channel on both tuners (for example recording back-to-back programs when you have Stop Recording = Extended or 5 minutes longer) that some times one tuner will have audio and the other will have no audio. Changing channel on either tuner usually fixes this (from that point onwards, too bad for anything recorded before I changed a channel). I'm not sure if the problem I described matches allanl's problem (probably not), but at least my problem is fairly reproducible (at least with ABC News 24 in Melbourne, but I suspect the same for other channels that need audio fix and for other locations).