They heard a small noise? How do you think that small noise would be interpreted by the two characters standing outside that room? You think the two gunslingers thought IT WAS MAYBE A FUCKING GUNSHOT? Especially the one who KNOWS SCHULTZ HAS A WEAPON THAT MAKES EXACTLY THAT NOISE?
Maybe it was a cake-related accident. Are you kidding me?
Seriously though - the editing in that scene is sloppy, because it actually INTRODUCES questions in the mind of the audience, questions that the text of the movie isn't intending to ask. Especially since it doesn't take "a few seconds" to process what happens. It's like THIRTY seconds. A full half-a-minute. Long enough for Stephen to go running across the room like "WAAAAAAAAAA" while the two gunslingers in he room just kinda goggle at what's happened as opposed to REACTING, which is what they'd do.
And again - if you're going to have them just standing there, you need to SELL that, visually, because there's no dialog to convey the info as to WHY they're just fucking standing there. The editing of the scene doesn't. It makes things confusing, and not in a story-appropriate way. It's slightly botched. Luckily, the gunfight gets going shortly afterward, so it's easy to forget the rough transition.