I've been using the program for a long time and I'm very happy with it - besides one thing. The sound level has always been really low. I can barely use the program without using my headphones, since the sound level is just simply too low.
Another thing you can look at:
In the Preferences of EarMaster, under the MIDI Instruments tab, set the "Attack" volumes for all playback instruments to 127 (the max value), and turn the volume slider up under the MIDI Devices tab.