1. An expandable and modular intercom and annunciation system, the system comprising: a plurality of signal sources; a plurality of originating interface modules, each originating interface module of the plurality of originating interface modules respectively coupled to a signal source of the plurality of signal sources, the plurality of originating interface modules for receiving respective signals from the corresponding plurality of signal sources and for converting the signals to common communication protocol signals; a receiving interface module coupled to the plurality of originating interface modules for communication via the common communication protocol; and an intercom and annunciation unit coupled to the receiving interface module to receive the common communication protocol signals, the intercom unit having a processor, having a memory, and having a user interface including an audio output; the processor having instructions to determine relative priorities among the plurality of common communication protocol signals, to select a first common communication protocol signal having a highest relative priority during a selected time interval, from the plurality of common communication protocol signals; the processor having further instructions to announce a message corresponding to the first common communication protocol signal during the selected time interval, and to maintain a quiet interval for an announcement entered via the user interface.