Mac or Windows? It probably makes a difference.

Like other platforms (Zoom, Teams etc.), you must be able to share your computer audio. This is different than your microphone. I don’t know Google Classroom but there are instructions for doing this somewhere. With other apps, sharing your screen should also share the audio but there are ways to share audio only. 

There is another way and that’s to output Finale audio. This is more complicated and probably unnecessary. For this, Mac v Win makes a big difference and the Mac is a lot easier. If you can get away with just sharing Computer Audio, that will be better.