This has had me incredibly frustrated on several occasions …but I think I just stumbled upon the solution…
Find the Multicam clip you have created, click on it and look in the audio inspector window. Scrolling down in the inspector reveals the Audio configuration setup and you can select the tracks you want for each angle – providing you originally defined your individual angles as dual mono …I think.
It took me hours…. and all I had to do was scroll down erghhh!!!