It's not the first time that I read about sounds problem, generally it happens using Windows Vista and 7 operating systems (no sounds only in menus after have launched) and seems also playing with the ripped MM2 copy at MM2BR (needs to run the setup.bat file).
After have installed the game, use the Widnows XP patch and the Speedboot, both available
here. Have no idea of what can cause the problem with your conditions, though.