Sound waves are not always invisible. Sound is simply the ordered vibration of matter (as opposed to heat, which is the disordered vibration of matter). Any time you see a vibration, you are seeing sound. For instance, hit a bell and watch it shake. That shaking motion of the bell is the sound waves traveling through the bell. When a jet flies close by over your house, you hear a rumble and see the silverware dance around on the table. That dancing motion is the sound waves from the jet traveling through your table and silverware.