This is the first time I'm doing such a project. I haven't started yet. Right now I have the raspberry pi in pic 1 (the original I think). If i press the button (pic 2/3) A signal goes towards the box (pic 4/5) and a sound is played. Is it possible to somehow connect the raspberry pi with the box, such that I can see when the button has been pressed and potentially do something with it if a signal arrived. How would be the best way to do this?
somehow connect the raspberry pi with the box
Let me see. I guess the box is a wireless door bell receiver. When you press the button of a box at the other end, the transmitter, a wireless signal is sent out. The receiver box gets the signal and starts its job, activates a buzzer, or plays music.
The buzzer is usually an NE555 like oscillator with audio/low frequency output connected to a power amplifier with output to a 4 Ohm speaker.
Now you can extract the electrical signal at the output of the oscillator, ie, input of the power amplifier (see below for a very simple example) usually 3V to 9V square/sine wave like like signal, convert it to 3V to 5V level if necessary, and use a Vcc = 3V3 Schmitt trigger logic gate, 74HC14 for example, to get a 3V level digital signal.
Rpi GPIO would now happily read the converted 3V digital signal.
If you don't want to mess around with the scary electronics stuff inside the box, then you can consider the connect by air or sound wave method, using a cheap sound detector. The sound detector listens to the door bell ring or music and outputs 3V/5V digital signal for the Rpi to read.