I was thinking of a thread like this.
Laspistol: A belt snapping.
Lasgun: The snapping sound of a whip. It sounds like a very hard, loud, slap to the face when it impacts.
Stubgun: A .45 pistol, or a revolver. Something loud and "barking".
Boltgun: The chattering of it's fire mechanism, the vhoosh of the rounds flying through the air on thin trails of heat and propellant, the teeth-shattering detonation as they chew their way into something. I imagine a semi-auto burst from a boltgun hitting someone would sound like a very loud, slightly wet, crackle as the shells went off inside their target.
Meltagun: A high pitched wine followed by something akin to jet engines. Anything struck by it would fizzle, pop, melt, and burn.
Plasma gun: A hiss of emergency venting systems as the low-pitched buzzing projectile is launched. Like a huge swarm of hornets.
Graviton gun: A low, almost out-of-hearing hum.
Chainsword: A low pitched but constant revving, until it hits something at which point it screams louder than any rock music concert.
Power sword: Imagine someone "dry firing" a taser (using it without it being pressed against something - it's very loud and very startling); like that but constant and much louder. A big crackle of energy that arcs from the blade itself, perhaps grounding itself in nearby walls with an alarming fizzle.
Krak grenade: Obvious really. A very loud cracking noise, like a tree falling over, but sped up a great deal. Maybe more like a snap.
Shuriken Launcher: Electronic, stacatto rattle. Actually I don't know much about shuriken launchers, so this is pretty much a guess.
Lascannon: Freem. Obivously.
Fleshborer/Barbed Strangler/Venom Cannon: I don't want to know!