how can they send you the letters when it's the neighbours, I would never live in a place that controls what I can and cannot do with my house

All you have to do is break into your neighbor's condo, steal her TV, electronics and jewelry, cut down the loud wind chimes and help yourself to a snack from her fridge. Making it look like a burglary will obfuscate the fact that it's just to settle a petty noise complaint. And you'll have some cool loot!

Why are they sending you letters when the wind chimes are next door? Just post the letters next door. Your neighbour won't know they ever went to your house and will either deal with the problem or cause problems with whoever sent them. Either way, you're off the hook.

I'm not sure what you're upset about. They're not your windchimes so you're not responsible. Problem solved.

You don’t know how to tell people they’re not your wind chimes?

Have you thought about calling the building manager and letting them know that they are the neighbors? Because that seems much much better than telling the inter-webs.