This is a new program for me and after my first attempt at editing a multi camera video I find that when I select a video source it also selects the audio and puts that on the output track. I need to keep the audio from one video source through out the edit.
However, the direct answer to your question is explained in my screenshot below. At the beginning of the track which contains the audio you want to use, choose the option Master Audio Track
This dropdown is accessed by clicking on the highlighted downward pointing arrow at the beginning of the track.