Converting your existing mp4a (M4A) audio file into an mp4r (M4R) file enables it to be used on your iPhone as a ringtone, as the M4R file extension is exclusive to the Apple iPhone. The M4A file format is free of Digital Rights Management and can be converted into any audio file format on your Macintosh or Windows PC. Once the file is converted into an M4R it can be synchronised to your iPhone through iTunes.
Open iTunes. Click "Music" in the side menu.
Locate the M4A in your music library. Right-click the file. Click "Get Info," "Options."
Enter the start and stop time in the appropriate field to create the length of the ringtone. Click "OK."
Drag and drop the file from iTunes to your desktop. Click the file once. Click the file once more while highlighted to open the file's editing configuration. Delete "m4a" from the end of the file name.
Enter "m4r" as the file extension and press "Return." Click "Use .m4r" to save the file. Drag and drop the file to the "Ringtone" tab in the iTunes side window.