I said No, but. Maybe a Masters would be easier? Do you have all the musical academic background you need already? Could you find someone in a music department you could collaborate on papers with?
The coursework before a PhD is ensuring you know the background, the PhD itself is proving you can do research, which you’ve already proven.
I’d think getting an academic job in music would be twice as hard as getting one in CS, especially in your specialty.
My daughter has two PhDs, but that’s because she invented a joint program and her university couldn’t figure out how to give her one in two departments, so it was easier for her to get two. Not the same as your situation.
This statement is key. Your first PhD proves you know how to do independent work. What would a second give you, as self-employed?
Forget about additional formal education, just do what you want to do. Your body of work will be the all the qualification you need.
Just an update:
I have applied to a PhD in music composition program. I’ll know if I’m accepted in late Feb/early Mar. I’ll let you know what happens, although the reality is I don’t expect to get accepted.
Why the sudden pessimism ? …
Sounded pretty hopeful back then.
I’m a musician now, I’m allowed (expected?) to think I’m the next Beethoven one day and the worst creature alive the next. 
I can’t really speak for the specifics of your industry, but as I understand it you will probably be turning 53 sometime in the future, whether or not you pursue this degree.