I had the same problem. In my case I had downloaded one of those "star wars sounds" apps and had set the r2d2 sound as a ringtone twice by mistake. I managed to delete one of them manually and then was left with the second "r2d2" file listed as a ringtone but it didn't play any sound. I tried playing with all the settings but couldn't manage to erase it. Eventually I just uninstalled the app and the ringtone name disappeared.
I dont know if that helps you any though. Good luck. Let me know if you figure it out.