I found a work around for this problem the other day. I had that issue myself for a while. I made a ringtones folder on the SD Card, threw the tones that I made onto it and assigned them. It worked at first but then everyone was back to the default.
The way I fixed it was I just put the full songs onto the SD Card, where there is the "Media" folder already set up. Then I played the song on the stock music player. There is an option to make a ringtone there, so I chose that. It let me cut it down to what I wanted it to be in the first place. It then asked if I wanted to assign the ringtone to a specific contact. I did that, and I haven't had the problem since. It works, locked or unlocked. Give it a try, if you can.