You can do all the necessary editing inside iTunes, Windows or Mac.
Shortly after the release of iTunes 7.4 -- which came out Wednesday, September 5, the day ringtones were announced -- users noticed that the main difference between ringtones and regular audio files was their file extensions. Regular AAC files have the file extension .m4a, while ringtones use the file extension .m4r.
• Locate your "Ringtones" folder. If you have iTunes 7.4 installed, there should be a folder in your iTunes music library labeled "Ringtones." ITunes 7.4 will automatically create a new folder in your music library for storing ringtones the first time you open a .m4r file, provided you have the "Keep iTunes Music Folder Organized" option checked.
• Edit the song down to a suitable size and length. The size of an allowed ringtone on the iPhone tops out at around 3 megabytes. For length, shoot for 20 to 30 seconds.
For Mac users, there are several ways to whittle down your tracks. You could use Apple's Quicktime Pro or Garageband if you have them. You can also edit MP3s in the free Audion 3 or the shareware app MP3 TrimmerAudacity.($11). Windows users can edit audio files with the free and cross-platform
Pick the 20 to 30 seconds you want to use as your ringtone and save the file as an MP3.
• Add the shortened file to your iTunes library.
• Right-click on the clip in iTunes and choose "Convert to AAC." The clip will need to be an AAC file in order to be used as a ringtone.
• Change the file extension to .m4r and move the AAC to the "ringtones" folder in your music library. Since both Windows and Mac OS X like to hide file extensions, the renaming part can be tricky. Make sure you aren't just appending a file extension to the hidden one. On a Mac, choose "Get Info" in the Finder and make sure "Hide extension" is unchecked. In Windows, make sure the "Hide extensions for known file types" option in Windows Explorer is turned off.
• Mac owners can use the free iRing Automator application to run this process in a single click.
• When you sync your iPhone, you should see the new ringtones displayed in your phone's sound settings.