It's so odd to me that you have all these random requirements. As a friend of mine memorably put it:

I'm doing an English degree, not building a house. Why do I need to take algebra?

Here you have to decide on a course of study at about the age of 15, which brings its own slew of problems, certainly, because you have to choose what to study in the final two years of high school. Not all subjects are compulsory in our equivalent of grades 12 and 13, so if you want to study chemistry or music at university you have to study chemistry and music during those two years.

So when you apply to universities you apply to the chemistry / music programmes at those universities, who then do the whole admissions thang and let you in or reject you, usually based on your predicted grades in those subjects. So, if you don't get the grades, no university for you.

(I realise you didn't ask about any of this but I'm here now...)

Once you actually get to university all of your classes will be related to chemistry or music. You might have space for 'elective' modules in your first year where you can study something unrelated to your degree, but even those are limited and will have their own separate entry requirements - it's not a free for all.

If, as some do, you realise 'oh, shit, I don't actually feel as passionately about covalent bonds as I thought I do, I'd like to do music instead' you'd have a very hard time moving from a chemistry degree to a music degree, you'd have to drop out and start the whole process over again.