In the guitar tabulature software TuxGuitar 1.2, is there a way to completely turn off sound or to adjust the loudness at which playback is done?
What you want to do is lower the master fader. Push Ctrl+M to bring up the mixer:
In the mixer you can adjust the volume of different tracks by moving the faders or mute totally by clicking the "Mute" box. To the very right you have the master fader. For some reason it's labeled "Gain" in TuxGuitar as of version 1.2. Lower it to lower all sounds output from TuxGuitar.