I had that problem in the past when I had my dishplayer. It only happend once in awhile in AC-3.
I had recorded on V-HS and watched the same part of the movie. The sound did not drop on the V-HS.
So it was a decoding problem in the AC-3. I ruled out my pioneer receiver because other people were having that problem as well.
Someone told me to upgrade my optical cable, but that was not the problem. It went away. I think it's a disruption in the signal going through the sat receiver.
I also have had a problem with my 4900. I would lose digital sound, and the only way to get it back was to do a reinsert card reboot. but that too went away.
I haven't had that problem with my 501, and I hope I never do
