So, let me try this again and hopefully I'll be corrected with my misinterpretations, hopefully I won't have many.
The audio is in AC and is coupled to TR1 via C1. VR1 sets the gain of TR1. Then C2 and R3 form a low pass filter which blocks certain frequencies that are amplified through TR1, which can't be estimated right now because there are no known values for R3 and C2, but will more than likely be in the Hz since we are dealing with audio? Do the coupling capacitors block the DC all together and only allow the AC component, which is the audio? Kind of confusing since I thought that AC couldn't be used with transistors?
Next, then audio is coupled from the output of TR1 through the coupling capacitor C3 to TR2, the next amplifier stage for gain. After TR2, C4 couples the output of TR2 to the voltage divider (R6 and R7) where the base of TR3 gets it's negative voltage, which partially turns on TR3 as an output amplifier. Now, R8 is bypassed by C5 for maximum voltage gain for the output of TR3 to the speaker? Why must C4 be bypassed here, DC is blocked again and only audio is allowed, the ac component?
I hope that I haven't made too much of a mess out of attempting to explain and understand the functioning of the circuit and what is actually going on in each stage of the intercom?
Also, why did the PNP have to be used rather than another NPN like the other two transistors in the circuit?
Thanks,
Matt