This is more of a enhancement type of idea rather than a gameplay helping type, as I do know that it's very easy to tell if you're hitting someone just by the fact that you can see their health bar lowering. However, I still thinking having some sort of beep, ding, bong (...nah) sound might be a neat addition to the experience of shooting someone and hitting them. One other issue that's been brought up before is the sound of friendlies shooting at you. Why is that still there? It's confusing when a friendly gunner is trying to shoot someone in front of you, and some bullets scrape you, making that loud klacking bullet-hit sound, and then as a result, you think that there's an enemy gunner behind you, causing you to turn around and possibly getting killed by that actual enemy that was in front of you.
I wouldn't mind some hitsounds, but the fact that you can see how much health an enemy has when you hit them makes it moot. See the healthbar? You just hit them.