Generally I do not think it is posible, I have tried myself with different sounds and the only way I could do it was to convert the sound to an ogg file and put it in the sounds folder. For a weapon you are in luck. You can get someone to fire it in the way you want like so:
murderer2 fire ["M16","Burst"]
murderer2 fire ["AK74","Burst"]
(I didn't know which wepon the guy has so he gets both instructions)