Hi and WELCOME to YT
! So glad you found us!
I have some suggestions for ya. First, discontinue the muzzle right away as it's a very confusing intervention in the way you're using it - it's sort of like teaching her that "play" is a bad thing and elicits punishment -- and you def don't want to teach her that.
Since your flat isn't very soundproof, I'd suggest either running a fan in your son's room and/or a noise machine like the one I use at night (omg LOVE IT):
Amazon
It sounds like your son is a *very* light sleeper and it would be better overall for the household in general if there was some kind of white noise for him to sleep to - it'd probably improve his quality of sleep too
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Do you think you could try one of those?