LukeFF, it's possible. Go to the properties of the wpn_Cannon controller in the sim file. Open up the Fire-node, and there you see a property 'effect'. It has an id. Select 'effect', and copy the value to the clipboard (in hex format!). Now, open up the particles.dat, and do CTRL+F, and paste the id into the textbox and hit Find. You'll see that the node will be selected either $MGun_muzzle_flash or $Big_muzzle_flash, or some others, depending on the gun. These are shared muzzle flashes by all (most?) guns. In the particles.dsd you will find the associated muzzle sound. To make a unique sound for a different weapon you have to clone a few nodes in both particles.dat and .dsd (to create a new effect+sound), a sound entry to the sdl-file (which defines the actual soundfile to be played), and lastly assign the newly created effect id to the weapon.
Hope this makes a bit sense
Just try
[edit] I'm not sure if there are tricks to do this easier (without cloning nodes), maybe someone else knows.