You will just need to make sure you include the classes needed for your music major on your 104R form you will fill out prior to contracting. The only concern the Army has is that you graduate in 4 years with at least on major completed, adding a second major will not effect your scholarship, just make sure you keep your 104R updated and work with the HR person in your battalion to make sure you are always on track to graduate. A double major sounds like a great idea since you have credits already built up, a lot will depend on just how many credits the university will accept when you start enrollment.
One tip, see if you can get in touch with your advisor at your new school to see if you can get started on the 104R form before the start of school. This will save you some time and allow you to contract sooner and activate your scholarship, provided you pass your APFT on the first try. Remember your scholarship tuition, book money, and stipend do not start until you have everything completed and pass the APFT, and the stipend is not retroactive, it starts when you contract. See if your battalion HR can get you as many of the forms you need to complete prior to getting to school.