ah yes...the dreaded converting xD xD...sorry dude...when I come accross this problem, I just download my songs on limewire...hehe...unless you are willing to spend 40 bucks on a converting program! Good Luck with that! xD xD
Xilisoft WMA - MP3 converter works well on .m4a files. You can get a free trial from here: link
For some reason though, I've never had to pay for my version, it's always been completely free (: