Need to know horror movie

I don’t remember a lot because it was a long time ago and I was a kid being babysat. All I remember from the plot is it’s a Horror/thriller about a detective or cop looking for something or a kids missing mom and the kid knows what happened but is too scared to tell them because they’re mute I think and the only thing else I remember is that there was this scene where they went into the vent and the mom was in the vent but she was like a ghoul or skeleton thing