Again the discussion seems to be missing a vital element, which is what she might want to do after University. Both maths and music seem to be being mooted because they are subjects she likes at school. Now a joint degreee in maths and music is being suggested in part because the maths entry requirement will be lower, eg no FM.
Several careers, including accountancy, barrister etc do not require a specific degree subject and a joint degree could, say, give you a head start in teaching. However it will presumably close others. If maths is the avenue you want to pursue then not doing the FM at school means you presumably spend part of your time at University covering the ground, and coupled with the limitations of a joint degree, means that you wont cover as much ground as peers who have decided on maths from the start.
I dont know too much about maths careers, other than jobs in the City, though assume others go into computing and elsewhere. I understand that it is possible to go straight to an economics masters if you have done the right sort of courses (presumably statistics type things) and a summer school or two, or you find a point where maths converges with another discipline, eg stochastics. My assumption is that the higher level the maths you have done the more choices you have.
I assume the same may be true for music. If you want to use a music degree in some way, you want as good a degree as you can get from as well regarded institution as you can find, then work hard and network.
For both I assume that the calibre of other students matters. DS' maths courses go at a cracking pace because lecturers know that everyone has have an A* in maths and at least AS in Further. (One piece of anecdotal evidence culled from friends with DC also in their first year University taking quantitiative subjects, is that DC at top ranked institutions - Oxbridge/LSE/UCL/Imperial/Warwick/etc - are working very hard. This suits most who have the capacity to work hard and play hard, and who are into their subjects. Though obviously hard workers are not exclusive to those institutions, general expectations in some other well-regarded RG institutions don't seem to be as high.) Really if she has the capacity to do either maths or music well, she might be better to choose now and go for it.
I think there has been a generational shift. Higher education is a huge investment. Overseas students, to a much larger extent, will be seeing a degree as a means of opening future career doors, which is partly why they are concentrated in specific, often quantitiative, subjects (though plenty in design type courses as well). I wonder if at some point, as well as hearing that good students are not getting into good Universities because they have chosen the wrong A levels, we will hear that good students are not getting top jobs because they have chosen the wrong degrees. I am not advocating taking a degree you are not interested in, simply because it sounds "employable", that would be a recipe for disaster. However I do advocate considering carefully which doors you want your degree to open and then making sure they are as open as possible. (Eg with law, get the best degree you can from the best University you can go to, and think a little about what you might specialise in, eg chemistry might help with patents law, maths might help with insurance.)