As promised, Nokia has released Ringtone Maker app for Lumia smartphones, but the app is currently available only for new WP8 Lumia devices and is expected to be made available for WP 7.5 devices soon. The app allows users to select their favorite music and set it as their device ringtone.
With Ringtone Maker you can create ringtones from the music on your phone. Simply open the app, select your favourite MP3 song and save it as a ringtone. You can cut your ringtone anyway you like, with editing controls that let you choose the start, end and length of your ringtone.
Nokia Ringtone Maker