From a Home screen, tap the Apps icon (located at the bottom).
Tap Messages. If unavailable, tap the Dropdown menu (located in the upper-left) then select Alphabetical. If still unavailable, refer to Manage App Icons.
Tap the appropriate message.
Touch and hold the audio attachment to save.
Tap Save as ringtone.
Tap Phone Ringtone or Contact ringtone. If selecting Contact ringtone, tap the desired checkbox next to the desired contact then tap SAVE.