Reference: http://www.pcadvisor.co.uk/how-to/audio/3274143/make-song-your-iphone-ringtone
1) Use audacity create the short (30sec) mp3.
2) Insert the mp3 in the itunes.
3) Right click create AAC version.
4) Delete both songs in the player
5) Look for AAC version in the music folder(iTunes).
6)Right click edit it into m4r
7)Go into Tunes and insert this song
8) Sync it and we are done.
