As I read in the book, you cannot use a silencer with las weapons.
Also, in the description of the several las weapons available, it doesn't say that the sound of a lasgun shot is less loud than a solid projectile (bullet) shot, although it describes it as resembling the sound of a whip.
Assuming that a lasgun shot is loud and not suitable to stealth operations, can't you use any device to supress the sound or the "flash" of these weapons?
From the picture of the long-las on the book, it seems that it has some kind of silencer, but the description says nothing about that. Also, on Gaunt's novels, Larkin uses flash supressors (and maybe silencers) with his long-las…