wav nor mp3 wont play as message alert tone

hi guys i need some help about my fone. i got my 6600 for 3 months before its playing wav as message alert tone then i oftenly use silent because of school. after trying back to set wav as msg alert, when i receive a message it doesnt sound up. i installed codecs for mp3 but still mp3 wouldnt play when i receive a message. i tried to put it the phone memory because im thinking maybe im using a 128 memory card and has lots of mp3 in it about 50 but compressed.. pls help me about this prob thanks