Just stumbled across the Audiko website – amazingly easy to do – just 6 steps…
- Upload MP3.
- Select section of the song, and if you want it to fade in/out.
- Download file – yes, they have an option for iPhone specifically.
- Import file into iTunes – the file will show up automatically in your ringtones list.
- Sync your phone.
- Go into your iPhone settings, and select the ringtone.
Enjoy!