Somehow everything is global. So a grenade thrown or some other loud noise on the other side of the map removes all footsteps on the map because of priority. So you wonder why the enemy suddenly shows in your face without hearing him and in a quite area - that's why.
I've no idea if it's possible to segregate the different sounds with distances and not everything being global but if there is such possibility please do fix it.
P.S. Ye I forgot - when this global sound event happens you also don't hear grenades thrown at you. I've died already more than enough times buy stealthy grenades.