How do I mute certain instruments during a song at various bars, not every bar?

Click on the bar that you wish to mute on, then press function key <F5>, then select the instrument(s) you wish to mute. Repeat at further bar(s) to bring the instruments back in again.

You can do this for all choruses, or only on selected choruses. There is a filter setting at the top of the form for selecting this.