It depends which course you're doing. If you've never done music before (although, I'm assuming that you did it in year 7 and 8, since it's compulsory), then I wouldn't attempt music 2, because you'll have a lot of theory to catch up on. Music 1, however, is actually pretty easy, and you don't really need a background in music, or even to be particularly competent at it. Only 30 or 40 marks of the total HSC are based on theory, and the rest can be any combination of performance, composition, and more theory.