Last week I lost the audio signals (both the standard right an left channels) as well as the optical audio feed to my Sonic Voom. Whats going on? Other PBS feeds have optical audio on AMC 21, it's just Montana PBS that seems to have lost it's audio.

Montana PBS doesn't have AC3 sound, that I am aware of. Can't check it for you right now but your receiver was probably feeding the PCM audio over the cable too, mine does on channels with regular pcm audio. Try deleting and rescanning Montana PBS transponder again.

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Yeah, it's definately not AC3, but that shouldn't matter. On the receivers I've tried, they would send regular mpeg audio over the optical cable too.
If it's lost both regular R/L and optical, and if it was working before, I don't think it's a situation of the receiver feeding to another port, since both were tried.
But it is likely that re-scanning will fix it.
I thought that perhaps they had changed the audio PID, but I just checked, and it's currently the same as it's been before, and the audio was working fine.

I'm curious though what receiver? I used to have problems with the optical output of the Mercury II. It just didn't work very well with some decoders, but my problem was that it would work with mpeg audio but not AC3, and this seems to be the opposite.
It could just be that the signal is weak, and has errors, but it's probably that the channel data got corrupted somehow, and re-scanning should cure that.