But this is (mostly ... in time terms larger than one or a few wave cycles) a spatial component where the signals to be added are kept separate until they reach their respective separate amplified speakers?
OK, so I need to just simply add the voltage numbers. How much head-room do I need? In the worst case I would need the coherent case. To do less I would need to understand the nature of my particular signals. In the case of a VoIP teleconference system (say maybe as many as 64 participants at one time), in addition to coherence being very unlikely, it might be rare for everyone to talk all at once.