If you press the "rec" button on the track you want to record your voice, then press the record (red button) on the center bar beside the play button, it will open a track record window. This will allow you to adjust the volume input and other setting for a voice record. When ready you hit the record button in that window and you should be able to record while the other tracks playback live. You can also mute each track independently if needed by pressing the mute button on each track controller.

Hope this helps.