AHA! Found this
You insert a DVD disk to watch a movie on a Windows Vista-based portable computer. You use Windows Media Player (Wmplayer.exe) or an application in Windows Media Center (Ehshell.exe) to watch the movie. However, the overall volume seems to be too low. For example, when you set maximum gain on all volume controls that are available in the audio mixer, system sounds and CD music playback are loud enough. However, DVD playback volume is still not loud enough.
This problem occurs because the default playback decoding mode of the AC-3 (Dolby Digital) streams is set to Normal mode. For DVD playback on portable computers, the default playback decoding mode should be set to Laptop Mode (RF decoding mode). The Laptop Mode (RF decoding mode) provides an additional increase in gain of 11 dB."
The article outlines a registry fix. Does anyone know if this applies to XP as well?