I am having the same issue. I've spent hours trying to get this solved, but if I have bitstreaming:HDMI set I don't get any sound on my tv channels, and my .mkv rips give an error that I don't have a good device. Audio works if bitstreaming is off. System specs:
Winxp pro sp3
biostar a55mh (hudson d2) mobo
amd a6 3500 APU
4g Ram (2x2g)
Hauppauge 2250 tuner card
latest ATI drivers (12.1)
PC HDMI => VIZIO HD TV => OPTICAL TO AVR - this setup works great on everything from wmc7 to openelec. I just need to get this set up w/ jpmc on xp.
BTW: I did a system restore hoping to correct this problem (tried MediaPortal prior to this, had codec packs installed etc..., so I restored to remove those), after the restore my trial period showed expired, I emailed for an extension and that immediately showed my 40day trial expired. I went ahead and purchased the software, because i was confident we can get this fixed, and I love everything else about MC. Any help is appreciated.