Want to isolate a few seconds of a MP3 for a free ringtone without buying and using an audio editor? There's an ability to do this natively in iTunes, but it takes just a little bit of looking around to figure it out. You can select any song in your iTunes library and create a separate file of the portion you want from the original file for use as ringtones, or perhaps some other uses you might have in mind.
Choose time to split
Load up iTunes. Select the file from which you want to extract a portion. Listen to the song, and watch for the exact time you want to extract in the status pane. After that is done, with the file selected, type Command + I to Get Info. Go to the Options pane, and type in the Start and Stop times for the portion you want. Finish checking the boxes and entering the values, and then click OK.
Creating Ringtone File
Now, right-click the file and choose Create MP3 (or WAV or any other format you have selected in iTunes Preferences, where you can also set the bit-rate, quality, etc.). When it creates the new file it will use the Start and Stop times you selected for yourself in the Get Info pane. Name the new file and export it to your cellphone by using the software your cellphone requires, or by using the native OS X Finder to drag-and-drop to your phone. For iPhone users, you must go to Finder or Spotlight to search for your newly created MP3, and rename the extension to ".m4r" and drag it to your User/Music/iTunes/iTunes Music/Ringtones folder.
Reset Original File
After you are done creating the small ringtone file you will want to go back to the original file, and choose Get Info, and uncheck the Start and Stop values in the Options pane, so that you will hear the song normally from start to finish anytime you play it again in iTunes.