As mentioned by Devin Lane over in this thread in the MacNN forums, you can use iTunes to trim off the beginning or end of a music or sound file -- permanently. First, add the file to your iTunes library. Then select it and choose Get Info. On the Options tab, you will see two fields at the bottom labeled Start Time and End Time. Modify whichever of these applies to your particular situation, then click OK. This will, as you probably know, cause the song to play from or stop at a different point than it normally would, but it doesn't actually alter the file at all.
The real hint here is the next part: in the Advanced menu, choose "Convert Selection to MP3" (if it says Convert Selection to something else instead, choose the MP3 Encoder in iTunes' Importing preferences; you can also use these preferences to re-save your sound file as WAV or AIFF instead). The converted file will be trimmed to the start and end times you selected earlier. You can then extract the file from iTunes using File -> Show Song File, if you're so inclined.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041230224327722