So are you on Stock ROM still? Or have you flashed to something else yet?
Anyways, where is the Ringtone file? Is it in My Ringtones folder, or somewhere else? If it is in the Ringtone folder, try moving it to Internal Storage. Not in any particular folder, just toe root of the Internal Storage. Then try to add it, not sure if it will work with that .dm extension, but thats where I have my Ringtones and they all work.
I'm still using stock rom but i'm gonna flash to a mighty. I stored it to internal and ringtones as well as windows but when i back out and go back to it the ringer changes to the first one on the list can't get the ringtone to stay
