Audio Track Buttons.
The audio track buttons are pressed to signify that
you want to use the audio recorded on the channels that are
turned on. These buttons reflect the current clip in the log
and will be copied automatically when the new clip or
copy clip button is pressed. The default is channels 1 and 2. If
you had recorded audio on channels 2 and 3 you would want
to turn off channel 1 and turn on 3. You would likewise want
to turn off all the channels if there is no audio recorded. (MOS)