The thing is-- nobody is checking up on your claims (at least not that we're aware of, anyway).
Basically, you just list it as an extracurricular in with the rest of it. It's not that big a deal, honestly.
I could have gotten a letter from her teacher, of course-- and if someone has checked into the veracity of the claim, we certainly don't know about it... and I suppose that there probably ARE candidates that claim a dozen years of viola or something, without any basis for it. At least so far, your word that you've done it seems to be enough; there is surprisingly little "documentation" required to "prove" claims made on college applications. The only deterrent to lying egregiously seems to be the idea that if you get caught, you'll have massively torpedoed your chances of admission anywhere but the local community college.
For MIT, I think that she had to provide contact information for all of her listed extracurriculars; she gave her teacher's name, studio address, and phone number.
Realize that this wasn't anything like a major 'hook' for DD, though-- it was just part of her personal landscape, like a regular long-standing community service gig that she also listed. She had probably four other activities that were as long-running and significant. Music is just part of the big picture for her.
