Optical holographic interferometry was used to observe standing sound waves in air inside a resonance tube. The resonance tube had a rectangular cross section and was driven by a small loudspeaker at one end. The front face of the resonance tube was constructed of Plexiglas, allowing optical interrogation of the interior of the tube. The object beam of the holographic setup was directed through the Plexiglas and reflected off the back wall. When driven at resonance, the fluctuation in air density at antinodes altered the index of refraction in the tube, causing interference patterns on the resulting hologram. Both real-time and time-averaged holograms were made of the first 15 longitudinal standing wave modes. The first eight modes agree very well with predictions for a closed-closed tube. This talk will focus on the holographic experiment and will demonstrate the visualization of standing sound waves.