QuickTime, a piece of software from Apple that is included in Mac OS X, is a nifty tool that can do more more than just play audio and video files. You can use Quicktime to trim audio files.
Open your audio file in QuickTime.
On the “edit” menu, select “Trim…” or press Command-T on the keyboard.
Two yellow bars will appear. Shift these around to find the segment of the file you wish to keep.
Press “Trim” when done.
You now have your shortened file. Remember to save!