We have our Console for Asterisk as our PA output here at our office. It is ghetto rigged, but it has been working a year. You can cut the end off a set of speakers (leave enough wire to work with though), then splice it onto a pair that runs to your PA (depending on the type of PA of course). Just adde the console to that call/conference/meetme/whatever, and you will be speaking on it.

If at multiple locations, just have an asterisk box at each location with a sound card and do the same.