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Defines the sound when hitting an enemy:

value description
0 no hit sound
1 default hit sound
2 multitone hit sound
3 reverse multitone hit sound

Enabled multitone hit sound will give you the possibility to hear how much damage you done to your opponent. It is a stepped sound, whereas a high sound means low damage and a deep low sound means high damage (when playing with cg_hitBeep 2). Combined with cg_crosshairHitColor 1 it is a handy way to estimate when you can "finish" your enemy with machine gun or shotgun.