[D1N0]

This is taken right from the Microsoft Windows Mobile How-To's

Save a sound file as a ring tone
If you have a sound file saved to the My Music folder on the device, you can set it as a ring tone. You must copy the file to the My Ringtones folder to make it available through Settings, where you can assign it to contacts or use it as a notification.
Step 1
Tap Start, tap Programs, and then tap File Explorer.
Step 2
Tap My Documents, and then tap My Music.
Step 3
Navigate to the sound file that you want, and then tap Menu, tap Edit, and then tap Copy.
Step 4
Tap Menu, and then tap My Documents.
Step 5
Scroll to and tap the My Ringtones folder.
Step 6
Tap Menu, tap Edit, and then tap Paste.

Hope this helps