I am finding the volume to be terribly low. I end up cranking my amplifier to MAX and still have trouble hearing quiet talking. I have tried messing with the Server Settings:
Settings/Web/Player/Audio Boost
Even at Huge setting it makes no difference.
All other apps and sounds in my 360 are fine. It's only when I try to play something on the Xbox 360. Using my Roku, I notice that .mkv files play quietly as well but not as quiet as everything on the Plex Xbox 360 app.
The setting for audio boost would have to be activated on the 360 and it does not as of yet have that setting available. I don’t have this issue but I also don’t have any 5.1 audio. All my movies and TV shows are 2.0 and are quite loud. You may want to check settings on your amp? How many channels of audio are you listening to?