The Elk-74 is a siren, rather than a voice speaker. The 3 leads are common, yelp and steady. Yelp is usually used for security and steady is used for the fire (I think).
The Elk-75 is a speaking speaker/siren with built in synthesized voice announcements.
The 73 is probably what you want as your basic combo voice and siren, but you can add the others.