To use your own tune it says goto Settings; Ringtones; Custom Ringtones, but I cannot see any Custom Ringtones, despite having saved a tun in the folder Ringtones. There is not an option - Custom!!!!
Nothing for load up an app, but I went to the apps website and downloaded ringtone maker - selected the file I wanted and it asks if I want to make that slice my ringtone, when selected yes, and went back to Ringtones + sounds; there is a custom heading and my ringtone!
It is a bit worrying that options are disappearing - I had this with my last phone Xperia J, downloaded the latest (as was) Android update and custom ringtone selection was no longer a possibility, had to do the same as this and install a ringtone app.
Check your mp3 file that it is in correct format. No longer than 40 seconds, 128 kbps and ringtone gender. Once I had 320 kbps ringtone and it didn't show at all as a custom ringtone. I changed it to 128 kbps and it works fine.