Of course we don't actually hear the sound of these acoustic waves. What we actually see is the pattern of the sound waves that is imprinted on the temperature of the CMB.
Compressing a gas heats it up. Letting it expand cools it down. The CMB is locally hotter in regions where the acoustic wave causes compression and cooler where it causes rarefaction:
Remember that in our color scheme red is cold and blue is hot! We will often use this alternate representation of acoustic waves as oscillating red and blue patches.
