It's great that you're interested but you have to be truly committed or even if you get in you won't stick. Attending an Academy is not something you just realize you want to try...you have to want it more than anything and be willing to go through a lot of difficult and exhausting trials from the get go. So think about why you want to go and how bad you need to be there.
Wome comprise about 200 some odd cadets out of each class as I recall but you aren't interested in averages, you are only interested in being one of the 200 plus.
Sports are very important. Every one is required to play a sport in the ACAD's as I recall, so it makes sense that they would want to see a candidate play sports in H.S.. Band is one of the activities that will earn you some points in one of the components of the composit scoring system.
Good GPA and solid SAT's are important as well.
Why did you suddenly decide so late that you want to deo something so demanding?