The easiest method is to just record your own ringtones. (Fab feature of the MDA).
1.)Find the music that you want to make into a ringtone. (it can be from the radio, a CD, your television, the internet, another device, or you can even sing it yourself. You can use anything that outputs audible sounds.)
2.)Go to the START menu > PROGRAMS > NOTES
3.)In NOTES, ensure you check the value >MENU > VIEW RECORDING TOOLBAR
4.)Once your recording toobar is open at the bottom of the screen, have your stylus ready and cue up your music.
5.)Hit the red circle at the far left to begin your recording.
6.)Record as long, or short, as you want it, and hit the black square next to the circle to stop your recording.
7.) Rename the file, and it is ready to use as a ringtone.
If you want it to be your main ringtone, just hold to select the file, and choose SET AS RINGTONE.
If you only want it for select contacts, open the individual contact, select EDIT, and tap the ringtone box. The new ringtone you created will be in the menu.