Although I've not implemented automatic volume control, of any kind, on this intercom, there are a few ideas that someone might implement. But the reasons I haven't are for simplicity, the circuitry is already designed so that it's maximum volume can't get too loud, and you'll probably be operating it fairly close to that level (so anything that's louder isn't going to be able to get significantly louder).
On intercoms where you may be working in a mixture of loud and quiet ambient noise, running a limiter to clamp down on very loud sounds can be useful. You could apply it to the party line, so everyone shared the benefit from a single limiter circuit. But as I said, with this circuit you've almost got the same effect by virtue of the voltage levels of the audio signals the intercom uses, and the low voltage of the power supply that runs it. It simply can't get very loud.
I did see a rather good feature mentioned on another intercom system. Whenever there was audio on the intercom line, it'd be used to duck the sound level on the program monitor (reduce the level, but not completely mute it). This allows you to talk over the top of it, rather than have to raise your voice to be heard over it.
This could be easily implemented in the base station, where the intercom line would be monitored, and used to drive a virtual variable resistor mid-way through the audio buffers for the program monitor feed to all the intercom stations.
Or it could be implemented in individual stations, where each station could optionally switch ducking on and off. For instance, the audio operator probably does not want to hear the program monitor dip in and out.
I find this generally to be a bad idea, because you hear the gain creep up during quieter moments, and all the background noise gets annoying loud. Then it suddenly changes when someone speaks. But the general idea is that the system would turn up the gain so a quiet speaker comes through a bit louder, and turn down the gain so a louder speaker wasn't as loud. This would have to implemented in the mike pre-amp stage of each station.
Personally, I feel a limiter circuit is a better approach. Clamp down on the loud noises, and do nothing to creep up the gain. Just get people to speak properly into the mikes.