We had A/V scope on a race track project recently that had an Emergency Communication System. In that instance, the ECS would take over every speaker and video display everywhere. All of the PA speakers, paging ceiling speakers, telephone paging, etc. were all overridden with emergency voice announcements. Likewise, all of the large LED boards around the track, digital signage, way-finding, etc. were overridden with emergency text messages. For us doing A/V, it added a lot of difficulty in that NFPA 72 dictates very strict requirements for mass notification equipment in regard to reliability, robustness, survivability, audibility and intelligibility, etc. For a typical A/V install, you don't have to worry about how long the system will survive in a large fire or tornado. For this project we had to think about that and plan accordingly. We had to think about protecting the cabling as much as possible, providing redundant systems where possible and putting everything on emergency generator or battery UPS backup power. In regard to audibility, NFPA 72 dictates that audible alert tones be 15dB above the ambient average or 5dB above the maximum sound level, whichever is greater. As you can imagine, designing a sound system that can produce levels 5dB above the engine noise of 24 race cars is pretty much impossible.
The consultant that wrote the spec for the ECS for this project was Hughes Associates.
You might try reaching out to them to get some basic info on what a typical mass notification system looks like.
Also, it wasn't part of our project, but I know that Mass Notification for schools and universities can also encompass sending text or voice messages to phones of faculty, students, family members, etc. as well as sending email messages.