Part of this is what you would be comfortable implementing. IFTT would be one approach.

There may be some overriding concerns, such as local fire codes. How would your system be notified of the message? How would the message be delivered to you? Will your scheme survive a power failure?

If you are using passive speakers one conceptually simple arrangement would be connecting to the house PA system through normally closed relays. The SONOS system would connect to the normally open side of the relays. In order to play the announcement, simply de-energize the relays. During a power failure, the speaker connections would default to the PA system.

Another thought: Is an emergency PA system already in place? Some fire codes will require this. In this case all you need to do is power down the SONOS system during emergency announcements. If appropriate, automatically restarting SONOS play will be a chore.