Does it happen to all speaker outputs, do headphones still work, when speakers loose the sound ?
If you have a DMM, it shouldn't be too hard to track the audio signal back from the output (plus with the schematic's) and see where the signal normally comes from. Then when it stops working, you should see it missing from those places.
If it's just analog audio in, and all outputs cut out about the same time, I'd suspect something like an Analog Input switch IC, or Volume control IC, or something related to them, has a problem. Assuming the power rails are all working, and it's not the actual output drive cutting out, but yeah it could be that too.
Or it could be some other muting function kicking in.