Don't remember if we can tell who is actually making those "atmospheric enhancements" but I remember anything that happens in the first few minutes well enough. I'm usually asleep by ten minutes in. And yes I love the movie.
Any way, it was to increase the creepy and weird atmosphere for the benefit of his guests. Same with the presumed cutting of the phone line "and I put it to you that this wire has been snip-ped in the last hour" (or something like that.)
He wanted his guests to get into the proper mood for solving a murder mystery. I think that explains making doors squeak and fake dust on the furniture and fake cobwebs anyway. The clock hand just seems to add a weird touch. Like the "storm only outside when inside" machine.