Sets the default ringtone for alarms, notifications or the ringer.
Music files you place in /sdcard/media/ringtones/ will be included in the selection list for the Ringer ringtone, similarly for Notification and Alarm ringtones. You may have to create the folders first.
An easier way to use SD card audio files as ringtones is to install a tone picker application e.g. Tone Picker.
You can also just put the title of the track in the box if it matches the title of an audio track you have on SD.