There are many ways to make sound visible, although this experiment forwarded by Bobby is a new one on me. Take some motor oil and pour it over a speaker cone that’s playing music. That’s just one of the things we see in this video featuring cymatics and music.
UPDATE: Since posting this, I’ve heard from several science teachers who use this video as teaching aids. Ian from Conestoga and Niagara College recommends we also look at this post.