In multicam mode (or after), how can I use for a scene not the master audio track, but one of the others? I have an audio glitch on the master stream that I want to work around.
Any audio track can be designated the Master, but I assume what you need to do is "briefly" substitute part of the current "master" with the audio from another camera?
Perhaps the "easiest" method of doing this is to cut the section of both the "master" and the "substitute" at the same point immediately before and immediately after the section containing the audio you wish to replace. For both tracks then unlock audio from video, delete (or move to a currently empty track so you can always put it back if you find the changed audio "doesn't work - see my caveat below - ) the audio currently in the "master" and copy and paste in its place the audio from the "substitute", taking great care to ensure it exactly maintains lip-sync, if appropriate.
One caveat; there is a real possibility that the inserted section will not exactly match the "ambience" of the original, with the result that any viewer of your video may detect an abrupt "change" in the audio for the period of the substitution. You will have to decide if this situation is "better" than leaving the original audio in place.
I also had to do this quite a bit in my recent project, and did it basically as Jeff describes. Regarding Jeff's caveat, I found that, after inserting the cut audio, it helps to extend it a bit to either side to create a crossfade. You may also have to tweak the level and apply some equalizer effect (and maybe others) on the inserted bit. For that, it may be easier to move the inserted piece into a separate track (holding shift to keep it at the same time), which makes it easier to manipulate using the mixer and effects (if necessary do the crossfades manually in this case). Finally, overlaying an additional music track or a low level "background noise" track across the entire inserted section (again fading that in and out) can further help to mask any inconsistencies.