Select a song from your iTunes library.
Click on the File menu to open up a dropdown menu. Select the option labeled "Info." A window will open, displaying information about the MP3 file and giving you a list of advanced options.
Click on the Options tab. In the Options tab you should see two boxes labeled "start time" and "stop time." The "start time" box should read at "0:00" and "stop time" box should read the length of your selected song. To edit the MP3 length, change the start or end time. For example, if you wanted a song to start 30 seconds into the song, you would enter "0:30" into the "start time" box.
Click "OK."