Hit sounds

[execut4ble]

Add a sound that plays whenever you do damage to someone. Just like in Reflex, Quake Live or Adrenaline Gamer 2.

Since there are no "critical" hits in this game, you could add a different sound for headshots instead.

If the damage is done through a wall (via wallgauss), don't play the sound since it would give the player some info advantage. Perhaps it could be played if the wallgauss hit is direct (if there's a way for the game to "see" that).

[Traderain]

You should do in a manner like in tf2

[chinese-soup]

This requires server-side implementation (or server side implementation of sending the damage dealt to the client). Or can be implemented using Metamod/AMXX, e.g. cutting it out of this: https://forums.alliedmods.net/showthread.php?t=281705