Below is a 2-station intercom using common 8R mini speakers. The “press-to-talk” switches should have a spring-return so the intercom can never be left ON.
Power the speaker from a separate power supply is the secret to preventing instability (motor-boating) with a high gain circuit like this. An extra station (or two extra stations ) can be connected to this design. [Source: talkingelectronics.com]