might have a free alternative idea for this. 

Use popup notifications for the announcement. You can set a duration in seconds in the console command for that. Let everyone know that the purge is active.

As "soundFX" you could do 2 options. The easy way and the uhm.. easy way i guess. 

1. Use the ingame system of the electronics. Give them a FREQ that they have to tune in with their own flashy lights and alarms. Inside your admin base ( presume you have one ) switch over that switch and let everyone know with some nifty soundFX of an caralarm.

2. If you mod your own maps, you could also make the same idea above, but solid without being broken or destroyed. connect it somewhere. like a nonconnected piece of a cave that no one can see as it's under the map. Build above your base and noclip down in case you need to activate the "alarm" for the purge. 

you could add alarms on all monuments you have, flashy lights and such.. however, i haven't done this before, but i know that it can be made.