Check the player position and only reproduce his shoot sounds to players in the same map position.
Exists a netprop that tell you the position in the map, like when you use radio commands that says your position, but i dont remember what netprop, I remember that I saw a thread about it. If not, you can use
my devzones plugin for make different zones and only the people on these zones can hear his shots. Or if not limit the shot sound by distance for example: distance > 1000 then block sound).
Plugin doable and not hard to do.
EDIT: ok the netprop for find the position is "m_szLastPlaceName"