The problem is that a suppressor (the correct term for a silencer) for a solid projectile weapon just fundamentally wouldn't work on a weapon that functions by super heating the air between it and its target.
As per wikipedia:
"suppressor reduce noise by allowing the rapidly expanding gases from the firing of the cartridge to be briefly diverted or trapped inside a series of hollow chambers. The trapped gas expands and cools, and its pressure and velocity decreases as it exits the suppressor"
Also a large number of weapons use special sub-sonic ammunition in combination with a suppressor to reduce the noise generated.
Now how does a lasgun operate? As per 40kwikia:
"The Lasgun uses the same basic technology and operates along the same lines as other laser weapons, emitting a beam of highly-energetic, focused, coherent photons. The high amount of energy carried by the photons of the beam causes the immediate surface area of a target to be vaporized in a small explosion"
Okay so we need to delve into how much kineti energy is in each lasgun shot, as it will effect how loud it is. Some digging on the internet brings up references to the standard lasgun bing in the "19 megathule range". This isn't a real measurement of energy but the closest comparison would be 19 megajoules.
Megajoules as per wikipedia:
"The megajoule (MJ) is equal to one million (106) joules, or approximately the kinetic energy of a one-tonne vehicle moving at 160 km/h (100 mph)"
So thats a 19 tonne projectile travelling at 100mph as the equivalent kinetic energy in a lasgun charge pack. It sounds like a lot but if you consider that a lasgun has approximately 60 shots per charge pack then its only 0.316 megajoules per shot. For comparisons sake a standard 7.62 NATO which is able to kill in a single shot has between 3 and 4 kilojoules of energy. This works out as a lasgun blast being 100 times as powerful as a 7.62 round.
Obviously there would have to be some major drop of energy over distance with a lasgun, otherwise that maths is far off and ill have to comeb ack to it..