There are no wind chimes in the basement of Clark's store at the beginning of the movie, and even though it's harder to tell, I'm almost certain there are still none there when Clark enters the Backrooms with Kat and Bobby. The first time we see the wind chimes is when Mary visits Clark's store. Kane even includes a shot of Mary looking towards them and then a close-up of the wind chimes. So why this emphasis?

Well the real question is... Where did they come from? If Clark's been in the Backrooms this whole time, do we really think he just nipped out to do a bit of decorating? Unless he was using them as some sort of alert system that Pirate Clark would trigger if he escaped the Backrooms. But that feels like a stretch because why not put them directly in front of the entrance instead of off to the side?

I think the real answer is that the wind chimes have started growing there. Like the red fuse switches, the wind chimes are another example of the Backrooms leaking into reality due to their proximity to a null zone. We see Mary pass through a room filled with wind chimes while running from Pirate Clark, and when she finally makes it to the copy of Clark's store, the basement has reverted to how it looked at the beginning of the movie, except the wind chimes are there.

This is the first example of reality copying the Backrooms instead of the other way around.

Wind Chime theory