I could probably come up with a dozen reasons why what you propose is not advisable. Emergency evacuation instructions should be delivered by a life safety system designed for the purpose when the need arises not by an IP phone system when a user decides to listen. In the event of any major catastrophy your network will probably be the first thing to cease working.

-Hal

CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.