Unable to assign MyMusic as Custom Ringtone

[chris22110fr]

Have Xperia S, Android 4.1.2 Build 6.2.B.0.200, since upgrade I have not been able to set personal ringtones for any of my contacts.  Prior to upgrade could edit ringtone in contact by selecting contact, ringtone and treble clef in top right corner to access my music files.  Since up grade this option seems to have disappeared.  Tried to assign ringtone to a new contact and all I seem to be offered were standard Sony ringtones.

Am I missing something, or is this a bug with this Build?  I use personalised ringtones a lot and would like to be able to do it again.

Any help gratefully received.

chris22110fr

[AlistairL]

Reset "app preferences" worked. All the other posters assumed that the + button was available. Thanks!