Are you deaf?
The chances are that you are lucky enough to be able to answer no to the question, but it doesn't mean that your hearing is crystal clear. Put me in a noisy room - a reception for instance - and I find it hard to differentiate the voice of who I'm talking with from the hubbub about me, even if he/she is standing within a hand's grasp. That nod and smile I'm portraying masks that I'm as much lip-reading as hearing what is said.
So where does this leave your characters? We live in such a visual world that many writers don't even consider using sound as part of their writing. They might mention that their character pulled up her collar against the wind, but then the wind is never mentioned again, and certainly not how it impacts on other sounds - if there are any.
As I sit keying in this article, the radiator behind me is slow-ticking, there is a dribble from the water tank in a cupboard, the gas boiler is on a burn in the next cupboard, my laptop's fan is spinning oddly, the wooden floor beneath my feet is creaking because a pipe-- Argh! It's a wonder I get any work done at all!
Yet most of this is ignored in my routine because... I'm used to it.
Don't become used to ignoring sound in your writing. Visualise (an ironic pun, there) where your characters are and what they can hear as well as see. A good exercise is to have them in a space where there is no light and they need to move from A to B on sound and touch alone. Close your eyes and try it in your house, but be careful. I'm not refunding insurance claims.
One of the stories dissected in Reading A Writer's Mind... focuses on sound. It was written both for normal reading and performance, and explains its use in detail.