What is the role of music therapy in treating depression?

Relaxation & coping. Usic that you enjoy will help you relax and be less up tight. Then it helps you to think more clearly and cope with your stress. If you can learn to do that itight start to lift your spirits if even only a bit!
As appropriate. Music can make you feel good, but can also make you feel sad. Appropriate music can be excellent help for mood disorders as well as general health. Inappropriate music can be very disruptive both for mood and health. Read musicophilia by oliver sacks.