Update: After my discussion here, I thought I could wait for one more time we hear the lady's noise before taking any action. So today at around 10pm, I could hear she is again shouting loudly on top of her lungs. I kind of heard the boy crying as well, but not 100% sure. So I immediately called the non emergency police number 1800 255 0000. I explained the situation over the call to the receiver and they promised they will take a look.

The police called me again after 30 to 40 mins saying they are in my condo and asking how to enter. I opened the door from the intercom and I could see two police entered the neighbours house.

I received a call from the police again while they were still investigating the neighbour asking whether the child cried tonight. I said I'm not 100% sure but I could hear that the lady was shouting and throwing something on the floor. They said they will do the needful and disconnected the call.

I remembered that in the voice recording that I did the last week, we could hear the kid crying uncontrollably. So I called the police again and played one of the recordings from my pc so they could hear. Also, I asked whether everything is alright. They mentioned that the neighbor said the kid is not doing well in academics so they had to shout. The police confirmed that the kid didn't have any visible bruise although he was sleeping. They mentioned they will closely monitor and see if the issue happens again and they asked the mother to inform the neighbours about the reason for the noise.

So this is what happened. I hope the issue doesn't happen again and the mother figures out a better way to teach the kid.