I certainly took my time but after long hours of tracing PCB tracks in Paint.net, identifying components and putting everything together in Kicad I finally ended reverse engineering the 2 PCBs there is in this video intercom.

To help me achieve my others objectives I then proceeded to identify which portions of the schematic where responsible for dealing with:

1. the audio coming from the entrance panel and going to the room monitor speaker
2. the audio coming from the door unit and going to the same room monitor speaker
3. the audio of the room monitor microphone and sending it to the entrance panel
4. the audio of the room monitor microphone and sending it to the door unit
5. generating the chime and sending it to the room monitor speaker but also to the door unit

For the sake of clarity, for each of the above I created a simplified schematic with only the components and connections that seemed to be relevant and compiled those 5 schematics in one big Kicad file (see FILES section for download).

Here is for example the portion of the overall schematic that deals with 1.

Thanks to those simplified schematics, I now have a pretty good idea of how this video intercom works and where in the future I should connect to in order to record the audio going between the entrance panel, door unit and room monitor. Also where to connect in order to inject a signal audio and have it played by the entrance panel or door unit.

While tracing back PCB tracks, I noticed that apart from the video signal, the remaining data and audio signals are going directly into a big IC branded with the manufacturer name (that I wasn't able to identify). Certainly a custom IC but next to it is the microcomputer (the one I referred to in my previous log) and after a close inspection, I found traces going only between those 2 ICs... so I guess they talk to each other?

I went through the microcomputer datasheet and checked the main function of each of the connected pins and it looks promising:

Next course of action will be to add some wires here and there so I can easily connect my logic analyzer or oscilloscope. Then probe for signals in situ, to confirm a few of my hypothesis but also learn what those 2 ICs are saying to each other.