I have similar issue with DVD audio and I have played around with at least 6 audio decoders. All but one have this problem. DVD audio volume is too low. The only one that doesn't have this problem is Sonic Cinemaster Audio (came with Sonic Cineplayer). I have tried ATI audio decoder, PowerDVD, WinDVD, Nvidia and the one that comes with JR Media Center - and all of them play DVD at half the volume.
Osho