change from CHAN_VOICE to CHAN_WEAPON
Edit:
(From Zombie Madness)
PHP Code:
emit_sound(id, CHAN_VOICE, sound, 1.0, ATTN_NORM, 0, PITCH_NORM)
Basically, Zombie Madness using CHAN_VOICE channel so the sound would overlap. I usually use CHAN_WEAPON for shooting sounds and CHAN_ITEM for other effect on the gun.