if you have a musical talent (like really talented in music) take it.
Go for Economics; it's easy and you don't have to do much work. A lot of it is commonsense. It's very well respected, even though it's not "required" for much eg. LSE puts it on their approved subject suggestion list.
I did A level economics and loved it.
I definitely think you should do music, everyone i know who takes it loves it, whereas many people find economics dry and mundane.
In terms of subject combintions, I think that music will actually look better than economics, even though it may be a 'stand alone' subject. After all, unis want well rounded people, so by adding an art subject to your sciences, you will be showing that you can do more than straight science.
Most importantly though, it sounds like you want to do music because you like it, so regardless of what your parents think, you will do better in a subject you want to do. So even if your parents think economics leaves better career prospects, a high grade in music will certainly do you more favours than a lower grade in economics.
Music is supposed to go very well with maths and sciences anyway. Apparently they all use the same part of the brain.
I'd personally say neither, Further Maths would be a good one as then you really do have all the well respected subjects. Plus if your doing maths and physics it will help.
If you do one (i say this losely, and comparitivley) 'doss' subject, it will worsen your application slightly, as it does no show that you know what you want to do and that you needed one more and thought what the hell.
music - if it is what you enjoy, then you will need it to keep you sane!
you only need 3 A levels for almost every university, and unless you want to take economics at uni, it's not worth it. most courses won't even specify which 3 you need, just grade/ucas point requirements.
check a few prospectuses if you have a course/institute in mind and see if they have any specific requirements