You can straight up run through the aliens, who forget about you once you're far enough (and by "far enough" I mean "once you're behind them.")
Mind: this video is the game on a higher difficulty—'Hardened'. The player is able to get through the entire level without firing a single shot.
I suppose the bright side to this is, if the awful shooting sound effects are driving you up the wall, welp. Maybe you don't have to shoot at all.
Aliens: Colonial Marines - Pacifist Mode [PCGamesN]