I can't think of a med school that wouldn't let you take classes at another college, but when would you find the time? Most med students will be in lectures most of the day, which is when classes at other colleges will be scheduled. I guess you're asking if you can take day classes at another college and if the med school will allow you to reschedule, and, well, that's not the way it works.
There's one lecture during one hour in med school - that's it. If you missed it, it's gone. Most places have a note service or recordings, but once the lecture is given, there's no afternoon repeat. Everyone has nearly the same schedule and you all take the same curriculum. You can skip class, and many do, but you will miss out on the lecture.
About extracurriculars, why don't you join a volunteer orchestra, if such a thing exists? If you've been playing so long, why do you have to be in a class to play? Aren't there any bands that meet at night, or a group of your classmates that are musically inclined?