These are the best audio settings for Modern Warfare 3 to help you hear your enemies footsteps from miles away.

Audio Mix

You'll want to head over to your audio settings and jump on the audio mix. You're going to want to pick between Headphones and Headphones Bass Boost. Now, as a general rule of thumb:

If you have a nice pair of Headphones. Stick with the Headphones.
If You're running something cheap, pick Headphones Bass Boost.

We help those footsteps up a little as certain. Sounds can get lost on Headphones without a good dynamic range.

Speakers/Headphones Game Sound Device

Just make sure you pick the correct device that you're currently using.

Volumes

Then the most important section on higher volumes:

Master Volume = 100

Gameplay Music Volume = 0

So this doesn't interfere with important sounds.

Dialogue Volume = 30

There are some important Dialogue cues in the game that you will want to hear. So that's why we don't want it completely off. But the majority of it will just be useless. So you don't want it too loud either.

Effects Volume = 100

These are noises such as gunfire and footsteps. So you want this as loud as possible.

Cinematic Music Volume = 0

Then on to cinematic music volume. This again is completely useless unless you're playing the campaign, so make sure is completely off.

Subtitles

Subtitles make sure this is off as well as this can just distract you from important visual cues in the game.

Mono Audio

Mono Audio = Turn off

This stops any surround sound effect and gives you the same audio, and both is eliminating any directional footstep and gunfire audio

Reduce Tinnitus Sound

Turn this off as adds another low bassy sound into the game, replacing that high pitched noise from concussion and flashbang grenades to a low dull and murky noise making it much harder to identify footsteps.