Because it does not insist on actual verbal communication, music allows the child the freedom of expression that can reach to his very heart and touch the essence of him as a person. Music is rhythmic which can be soothing to everyone even those who do not have autism. Patterns and repetition can also make simple songs easy to learn and to remember. The ability to make sounds that echo what the child is hearing inside may also be a saving grace because the ability to communicate is being restored. Exploring simple instruments can also encourage children to tolerate different textures and materials. After a few minutes of time it might be easy to see what kind of tones and sounds are positive to the child because those will be the ones that they return to frequently.
While music can certainly work for many children and even adults with autism, it is important to remember that it is not a magical "cure" and that not everyone will respond to music therapy. Those who enjoy it will likely see many benefits though and that alone makes it worth exploring as an option especially when other therapies have failed to produce any results.
Music therapy pairs well with speech therapy. As the child accepts music's rhythms and patterns, simple songs can be taught. Eventually the words move from the words of songs to the spoken word and can be progressed from there. Possibilities can be endless and all progress, no matter how small should be considered a victory.