This is a 2-station simple intercom circuit build based on transistors part and using common 8R mini speakers.
The speaker works as microphone and generate sound, so there is no need microphone for this intercom.
The "press-to-talk" switches should have a spring-return so the intercom can never be left ON, push on switch can be used for this kind of task.
The secret to preventing instability (motor-boating) with a high gain circuit like this is to power the speaker from a separate power supply! You can connect an extra station (or two extra stations) to this circuit design.
Build the circuit into two separated boxes (for 2 stations). On each box should be use 2 ports for external power supply port (if you use external power supply, not battery) and connection port to other stations. Create some holes for speaker push on switch.