Long time ago I got the knowledge that iPhone did not support customized ringtones. This is not a fact anymore. Here is how to make a customized iPhone ringtone.
Import your audio file into iTunes;
If your music is longer than 40 seconds, you need to do this step. Otherwize you’re OK to the next step directly. iPhone doesn’t allow a ringtone longer than 40 seconds, so we have to set the length of this file to 40 seconds. Here is how-to: “Ctrl+click” the imported music file in iTunes to open the right-click menu, select “Display brief Introduction(显示简介) -> Options”, set the “End Time” to 40 seconds, done;
“Ctrl+click” to open the right-click menu again, select the “Export to ACC format” item, then you will get a 40 seconds ACC audio file right below the original one;
“Ctrl+click” the new file and select the “Open in Finder” item, in the opened Finder window, change the file name from “something.m4a” to “something.m4r” and confirm to save;
Import the “m4r” file into iTunes(you can drag and drop this file to iTunes);
Connect your iPhone to your computer, open it in iTunes and select “Sync ringtones”. If everything works, you can select this ringtone in your iPhone now.