Please see NCH's BMS Software here: http://www.nch.com.au/bms/index.html (Music and announcement system for stores).
However, there is an alternative option, and one that is more voip compatible:
I have managed to do this, however only with the aid of NCH's IVM software. Create an extension that IVM is setup to answer right away. Have it answer with an OGM that goes straight to a given voice mailbox without playing a greeting (with an optional beep or something played over the external speaker system). Then make sure the call screening option is turned on in the mailbox properties.
The result is a system that allows you to phone an extension which is answered pretty much immediately and then plays a short tone, after which your voice is broadcasted over the PA system (speaker system connected to the computer). There may be a slight delay, but in some ways that works out better in the end because it reduces the possibility of feedback (when user's voice enters Mic (phone) comes out speaker system loud enough that it comes back into the Mic (phone) and continues to loop leading to high pitch noises that leave listeners feeling very uncomfortable).