From a Home screen, navigate: Apps > Messages. If unavailable, tap the Apps dropdown menu (located in the upper-left) then tap Alphabetical. If still unavailable, refer to Manage App Icons.
Tap the message.
Touch and hold the audio attachment.
Tap Save as ringtone then select one of the following:
Phone ringtone
Contact ringtone To set as contact ringtone, tap (check) the appropriate contact then tap Save.