The mixer is the easiest way to adjust volumes for the various parts while listening to the result. When you press ok modifications will be saved. The first fader sets the tempo.
Each voice can be disabled/enabled for playback using yej corresponding check box.
The horizontal sliders provide panning/balance: the 'position' of a voice within a stereo setup.
The voice instrumentation can be changed using the instrument button
On top of the mixer you'll find start/pause/stop buttons as well as a MIDI-fine-tuning-knob to accommodate a possible out-of-tune instrument in playing along. This way you can alter the A4 tuning from the usual 440Hz.
The blue mixer-button on the bottom takes you to the windows mixer where you should make sure that the MIDI-volume is non-zero.
Another place wher you can set audio levels is the score-dialog