Our bodies are made of moving energy, as is sound. Sound is a wave of energy, a frequency. By using sound on or near the body with intention, we can help the body come into resonance with the intention we are holding. (This is really a simplified explanation, but I think quite effective). According to Steven Halpern, noted musician "Sound is a carrier wave of consciousness". Edgar Cayce (the "sleeping prophet"), predicted that sound would be the medicine of the future.
In a sound healing session, I may use various tuning forks on various acupuncture points on the body, chakra areas, or in your subtle energy field. I may also use Tibetan bowls, bells, tingshas, drumming, and voice to move you into a relaxed state (sometimes called theta state, although you are always in some combination of states). In this state your body is more receptive to finding its natural state of healing and ease. Here is where the magic happens!
Here’s an excellent article on The Therapeutic Use of Sound in Alternative and Conventional Medicine, by Eileen McKusick (my Biofield Tuning Teacher).