Hooray a chance to practise my masters degree topic
Well... erm, copyright subsists in all music as you probably know. By arranging someone else's music you are technically infringing on their copyright. Your arrangement will actually carry with it a copyright of its own, so if someone copies your arrangement you could sue them :p But that's totally off the point.
The point is that while the copyright holder will be entitled to stop you, they generally won't because firstly they probably won't know, and secondly it's just not worth the hassle for them (thirdly, many copyright owners don't really care). If you start making big bucks from it then they'll see a chance to make money off you, and then you'll need to think more carefully about it.
But generally speaking you're fine
If you plan on making money from the arrangement then you're best off getting the permission of the copyright holder (usually by buying a licence from them).