hello i have the Droid Ultra and I want to know if I could use a song from my music library as a ring tone?
Save a copy of the file in the Ringtones folder of your phone's internal memory and you should then see it as an option when choosing ringtones. You can likewise save files in the phone's Alarms and Notifications folders if you want custom alarm and notification tones.
Most android devices and iPhone's have the ability to use your own music as a ringtone. I use multiple songs for individual contacts on my wife's LG G2 and my own iPhone's.
Your owners manual should tell you how to assign music as a ringtone. or just do a Google search on the process for your device.
http://www.google.com
Good Luck