download Ringtones! app and follow the instructions to make a 30 seconds music. Sync your device with your PC and open Applications >> Ringtones! and transfer your new ringtone for your PC. Then sync your device.
The sound will be appear in Settings >> Sounds >> Ringtone, and select your customized sound.
Remender: Ringtones must have up 30 seconds, no more than that.
Hope it will be helpfull