I haven't heard of anyone trying this trick on the a900, but take an mp3 and give it a false extension. I play mp3 ringers on my phone and it's not even supposed to support mp3 files. Well it depends on your windows configuration, can you see the ".mp3" at the end of the files on your pc?
Try erasing the .mp3 and change it to something else your phone supports, .qcp works for me but I've heard of others doing .wav or .mid, it's worth a try but PLEASE let us know if it works or not! If you can't view the extensions of files, I can tell you how to change that.
