We've had that suggestion before, but a lot of different reasons prevent this.
It's a scientific fact that the human brain is faster with visual then with acoustic signals.
This means you'll notice a delay in graphics much better then in sound.
Having the hit sound played 100-200ms after the hit won't be recognizably different,
but having a blood effect display delayed for that time will be.
So when your client is sending a signal to the server and it takes 100ms, then the answer
possibly takes another 100ms which results in 200ms delay.
And i've seen ppl playing and i played myself on even higher latency then this.
How would it feel like to see a blood splatter on a oponent a second after you placed the last shot on him?