The mic is a series of strokes, and the circle is a drawn circle in sketch. I am trying to make the mic symbol transparent with the rest of the circle filled in. How do I do that?
Sounds like what you're looking for is the boolean subtract operation — I would highly recommend reading the linked page since it's a very valuable resource.
You might get some weirdness since the microphone shape is a set of paths rather than a combined shape of its own, but you can correct that with
Convert to Outlines before using the subtract operation.