I think I ought to clarify my last remark here.. I'm assuming you're adding a music mix to your clip. if so you might've tried to insert it on the narration track(which has a 'speaker' icon). In some, but not all cases, the narration track may not recognise MP3, but the music track(1 only in PD7, 3 or more in PD8 and upwards) will, or should accept MP3. If, like me, you might prepare a narration track for your video, if the narration track won't accept MP3, then have a WAV file version of your narration ready, "just in case!". Or put your narration track(MP3) on one of the music tracks(PD8 and upwards).
Cheers!