You've got an amazingly talented young adult BN - what a delight she must be. This blip looks to me like an understandable reaction to a time of intense pressure. Nobody mentions how difficult A-levels are. If she can do what she's done, she can do anything she turns her mind to. And just so you know, mathematical and musical talent often go together, so being good at music isn't a million miles from having a good head for calculation. You can reassure her about this little point.
Part of her dichotomy might be that she's realised the treadmill she's been on. Most sixth form colleges channel students to universities - it makes their own figures look good - but it isn't always the right choice for the individual.
So she might do herself a favour by taking a year out. She could do some music-related work, or internships, or work-shadowing - maybe if you're in the right location she could even get a paid job to tide her over and gain some work experience. I'm not at all suggesting future routes, but the reality is that many well-qualified graduates in the UK can't get work commensurate with the sweat they put into getting qualified. So supporting her in taking stock of what her options could be for the future might result in her deciding not to attend uni at this point - which need not be a major disaster.
Does she have friends, a social group, attend a hiking group - anything to take her mind off the here and now, as the lead-up to results day is agonising, and I'm afraid there are idiots on social media who just whip up the hysteria.
Just your solid presence as a parent and unquestioning love are very important, now as ever, and after results day she might feel the pressure easing.
But really and truly, she deserves some time without academic pressure to reflect and reassess, and the best of luck for the next few weeks. Let us know how she gets on!