I have run across a few musicians who have put some of their stuff in the public domain under the Creative Common License. There are some other sites out there that you can pay a fee for and then use however you wish, but these guys give their stuff away for free, as long as you include them in the credits. The links I have provided are to their license agreements that state you are free to use the music however you wish.
The first site has probably the best music for use in M2 or RTW, but all have some pretty good stuff.
http://incompetech.com/m/c/royalty-free/faq.html
http://www.jimmyg.us/Home/tabid/54/Default.aspx
http://www.jmtr.com/jmtr-royalty-free.html