Record USB audio in sync with Facetime Camera on FCPX

Hi!

So I'm trying to record video of a solo musical performance using the in-built Facetime Camera on my mid 2012 MBP, but using an external USB audio device. I know I can use the "voice over" feature to add the audio in afterwards, but that then leads to an endlessly irritating fuss of syncing it all up together..

Is there a way (which I can't currently see) to record external audio from the screen where I import video from the Facetime Camera? Or is there another way of doing it altogether?

Thanks!!