While sound is not nearly as dominant, it's absolutely not just a backup sense. It's the fastest perception we have (the best rhythm game players can play blind but not deaf), it covers all directions, and even in our sleep we still respond to loud sounds.

Sound perception is so fast that it's often what directs you to look in the right direction, even if what you're reacting to happened in your field of vision.

Funny enough, even our peripheral vision is faster than our central field of vision, to help us avoid predators coming from behind! Our forward directed vision is for tracking and understanding what's in front of us, sound and peripheral vision is in large part for environmental awareness. They're co-dependent!

Humans can even learn echolocation!