First things first, well done! I know you're disappointed because it's below your prediction, but an A is an excellent grade, especially in a subject with grade boundaries as high as Music.
In terms of advice, it's worth a shot. Firstly, I'd check how many raw marks you need in the paper to get the extra 3 UMS marks - it'll help you to judge exactly how likely your A* could be.
3 UMS marks off is always worth a remark; just in case. Obviously, the usual 'don't expect anything from it' and 'prepare to lose your money' warnings apply. Good luck.