http://www.networkedmediatank.com/showthread.php?tid=31092 < These folks had it too, seems a firmware upgrade helped them out too.
Thanks Jackal. Unfortunately, it looks like this was a slightly different issue, since those peeps were getting hissing sounds and did not really mention an AV receiver.
The new firmware did not do shit, whether I set the HDMI sound to mute or not. Once again I got the sound for all the previews (which is apparently because they're never with full-blown HD sound), but nothing during the movie. Since I don't really feel like buying a new blu-ray player or surround sound system, I think my last resort is to stop using the HDMI cable from the blu-ray to the TV, and use the regular RCA analog cables instead, but the visual quality will obviously not be the same...

EDIT: just saw this, sorry to hear about that Pdub, hope everything will be okay.