The quality of mp3 is one of the lowest out there.
Just convert it to anything (aiff, wav, sdii, preferably at 24 bit, and at 44.1 kHz as mp3 is at 44.1 kHz)

mp3 to wav

edit

wav to mp3

Why start out with mp3s though? Quality is not that good

i think a 320 kbs mp3 is of equal sound quality to a cd....