My DS is doing Financial Maths, which is kind of a "joint" degree between Maths and Business (economics, finance & accounting). He says he wouldn't have done it had he known how it would be run.
For a start, he doesn't feel any sense of "belonging" to either department as it's very close to 50:50 between the two depts (slightly weighted towards Maths to get "Maths" in the degree title!). He says that the Maths students tend to be a close knit group as they're doing Maths most of the time so have far more "common" lectures etc so they're a difficult group of people to get in with. Likewise, his "business" lectures are also the same as the business/economics/accounting students do more common lectures, so again, they're a hard group to get in with. So, basically, most of his lectures, seminars etc he's the odd one out. He says very few students are doing the Financial Maths degree, hence very few people doing the full suite of the same lectures as he is doing.
The course admin is also poor, i.e. clashing lectures, clashing exams, etc as the two depts don't seem to liaise with each other. On one particular day, he had 3 exams on the same day, 2 of which clashed directly so he had to do that in the evening. Both depts had previously said that they'd only do a maximum of 2 exams per week over 6 weeks to help spread the burden of revision and stress of exams, but that didn't apply to joint degrees.
None of the above issues really manifested themselves in his first year (20/21) as it was the covid year and everything, literally everything was online for the full year, so he didn't really have the same problems. But certainly, 21/22, his second year, he expected things to be better, but really, they weren't.
As for module choice, he basically had none in the Maths sections, all modules being compulsory, and unfortunately, most of those were what he regards as the boring and "hard" maths, i.e. the theoretical stuff, whereas he prefers real life Maths. Even the probability and statistics modules were more "theory" rather than real life, and unlike either the Maths or Business students, the financial maths students didn't have any option of doing any computer-related maths at all, no programming, etc., as there's simply no space in the module choices. All his options are within the business half.
Obviously, a lot of the above is irrelevant as OP refers to Maths and music, but I think the maths points may be relevant as the OP's DS is doing a Maths degree, so may find that module choices in Maths are pretty limited -definitely something they'd need to check before committing, i.e. just how flexible are the module choices!