There are a number of audio applications that allow you to edit mp3s, but Quicktime will also do the trick. Make sure you have the latest version, as some older QT versions don't handle mp3.

http://www.Apple.com/quicktime/download/

1. Drag the mp3 onto Quicktime Player.

2. Drag the two little triangular selection points until they mark the beginning and end of the part you want to extract.

3. Press Command C (copy selection)

4. Open a new player window (command N)

5. Paste the extract into the new window (command V)

6. Save this new file.