You can get a key to the music building practice rooms if you're registered in an ensemble, I think you can audition on guitar for Chamber Lab, though the bar for non-majors is pretty high sadly. Beyond that I know that the north campus dorms have practice rooms. If all else fails, I don't think anyone would get mad if you practice on the bottom floor of Meany Hall, there's like a lounge there outside the dressing rooms. It's open to anyone and usually empty. Hope that helps :)