I played some Frontlines multiplayer the other day with some fellow CoWs. Somehow, Oak managed to sneak up on Scott with a tank. Yeah, let that sink in a moment.
He was probably able to do so for one of three reasons: (1) Scott was daydreaming, (2) Scott was so focused on something that all else fell out of focus, or (3) the sound of the tank was masked by other sounds.
It's the third possibility that I think designers should think hard about, because it's a trick rarely used. Sound doesn't have to be a subconscious element in combat. It can be a factor in difficulty... something the player must rely on or distrust.
For example, there's such thing as audio camouflage - intentionally mimicking a sound to create a mistaken impression. It can be used to hide dangers or to make the harmless unnerving. It's a power that can be limited to AI and environments or offered to the player as an active skill.
Noise can be distracting and disorienting. I don't recommend setting an entire game level around loud machinery, but the sort of constant noise you hear next to planes or jets can hide enemies.
The cocking of a gun can warn players that something bad is about to happen... or tip off enemies. Imagine sneaking up behind an enemy and cocking your shotgun just so you could see the enemy jump with surprise and scramble for safety.
My point is sound has a lot of strategic potential in games that rarely gets used.