Assuming the audio was recorded discretely into channel 1/2 on the main camera- i.e. is not a stereo mix, then you should be able to access the audio components on your Project timeline.
Do this by selecting the Multicam clip, then Clip>Expand Audio Components. You’ll then see a representation of all channels in the currently active audio angle.
Assuming you switch only video and leave the audio from the camera that has the lav/other input as the “Active Audio Angle.”
My suggestion would be to do a first pass for video switch only with both channels on. Then when you have the picture switch as desired, go back into the audio components and set the levels as you wish. And remember audio component edits can extend beyond the video cut point. So you should be able to do this all without any sort of detaching of audio/mucking directly in the angle editor.
For a much more detailed explanation-
https://www.youtube.com/watch?v=4RlQOqrLV-w
Noah
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