With the ChevronWP7 custom ringtone manager you can upload your own ringtones to your WP7 device.
Just start the tool, select your sound files and generate a XAP-File. The deploy this to your phone and start it on the phone.
Now your ring tone is available on the settings.
http://thetechjournal.com/electronics/mobile/chevronwp7-ringtone-installer-has-releasedhow-to.xhtml