Sound travels in waves of pressure made up of compressions and rarefactions (the opposite of compression). If you were to stroll about a large room as speakers played music from a stage in the front, you'd encounter areas where the music was louder or softer, as the waves cause interference with each other.
The spots where compressions meet each other are louder. Areas where rarefactions collide are softer.
And where compressions and rarefactions smash together? There's little to no sound at all. When architects design concert halls for musicians, they must carefully consider the acoustics of the building. An improper design results in dead spots where sound waves cancel each other out.
This same principle works in noise-cancelling headphones. These headphones detect incoming sound (like a baby crying on an airplane) and create opposing sound wave, which eliminates the cries and lets you enjoy Mozart instead of, "Mommy!"
In a concert hall, to stop interference and dead spots, engineers often install padded walls or panels that absorb sound waves. These panels reduce echoing and thus the weird interference that would ruin a listener's experience.