i had the same problem, couldnt set an aac or mp3 as ringtone.
the problem with me was realplayer advance.i installed it on the mmc and after i uninstalled it my ringtones came back!
u can try that or if ou have another software which uses your mp3s u need to get rid of the whole thing
thank you very much....
i removed realplayer4.0 and my mp3 seems as ringtones