[[ As Consultant already stated Garageband comes free with your Mac. ]]
There could be a problem if the user is trying to edit compressed files, such as mp3.
GarageBand will edit them, true. But it first has to de-compress the file into aiff format.
That will be ok -- so long as the user doesn't want to "re-compress" them back into mp3 format. To do so will "throw out" more audio information (because of the "lossy" compression process), and could degrade the audio quality.
MP3 Trimmer is shareware, but all the features are available in demo mode.
It can trim and modify mp3 files WITHOUT de-compressing them, thus retaining the original audio quality.