I am having a similar problem I think. I have all of the volumes set to 100 and have the packages installed in the right places. The music and sound effects play fine, but all of the voices have very low volume. The only way you can hear them at all is by turning the music off.

This is on Windows7 32-bit with a Realtek on board sound chip. It is the same with the OpenAL and mixSDL sound drivers.