I'm sure they are addressed somewhere in canon. But, I'll leave that to the canon scholars.
What works for me IMTU:
ACR & LAG: They're firearms, just as today's. As loud as any other smallarm.
LAG louder, since it's basically an Anti-Tank Gun.
Accelerator Rifle: Discharges it's round at a low velocity (to reduce recoil), then the round 'speeds' up.
Well, as far as initial muzzle velocity goes, that's pretty much the science behind some "silencers", which reduce muzzle velocity to sub-sonic speeds, thereby reducing the crack of the sonic boom report.
So....initially, I would say they are as loud as a 'silenced' firearm. Then, while the round propellant (mini-rocket) ramps up the speed, I would attach a sound to that. Kind of a quick, high pitches 'whip'. Maybe akin to what 4 or 8-ounce firework rockets sound like.
Laser Weapons: I use slighly different sound levels, depending on whether they are of the 'power source' or 'CLC' variety. It could be argued that they are virtually silent, but my preference is that they make some sound, albeit not near as loud as a firearm.
My personal SXF reference sound? Movie Terminator: Nifty little sound in the future-flashback(forward?) that the Hunter-Killer aircraft spat out. "bzztt!"
DB level similar to a silenced firearm; increasing as the MW/Power of the Laser increases.
Gauss Weapons: While there is no chemical reaction/propellant to add to the sound, I feel the ultrasonic speed of the projectile is still going to give you a report as it exits the muzzle. IMTU, gauss weapons give a report similar in db to a .22 CAL pistol round. More than a suppressed round, but pretty close to a typical fire-cracker. As far as what it sounds like, the Mechassault game on Xbox has pretty kewl sound for gauss weapons (and several others) that my group uses as a common reference.

NOTE: Keep in mind that, for the weapons that CAN fire in airless environments, they WOULD effectively be silent, since there is no atmospheric medium for the sound to travel though.
Hope that helps. I just went with what I thought made common sense.