That's right.. and it works the same for humans. For the brain to be able to locate a sound in all dimensions, it needs two separate points to compare.

More info here on reddit

Having two ears gives you the ability to measure the angle of a sound relative to the axis joining both ears. Unfortunately this means that if you're holding your head still you will have difficulty distinguishing whether the sound is in front, behind, above or below you.

Echoes from nearby objects may give your brain the clues it needs in many situations but when you just can't seem to find the source of that noise, try this:

Tilt your head side-to-side in the shoulder-to-shoulder direction while listening. As long as your head is moving along that path when you hear the sound, you'll get a better idea of where the sound is. But remember that this doesn't tell you whether it's in front or behind you. For that, turn your head (like the "no" motion) while listening.

Of course, all this wouldn't be necessary if we had a couple extra ears in odd locations. ;)

Depressing that not a single dog owner in this reddit seems to understands why their dogs move their heads like this.. :/