there may be something wrong with the mp3. sometimes i receive mp3 files from clients that i can't import into flash. when that happens, i drag the mp3 into iTunes, select the file, click advanced, click create mp3 version. iTunes creates a new mp3 file which i click and drag out of iTunes (which creates a copy of that new file) into my job folder. flash imports the mp3 i dragged into the job folder. then i go back and delete both versions out of iTunes.
note: if you don't see that option in iTunes, go to iTunes preferences > general > import settings > mp3 encoder