(still an early draft)
A voice board is like a discussion forum: where people can leave voice messages (in 'threaded' form) ..
Wimba is the most well-established, but certainly not free .. except as part of the TafeVC.

examples include:

apparently you can build your own board in flash: tutorial here: http://www.tutorialized.com/tutorial/Creating-A-Sound-Board/10714

more info about live conference tools at Community Engagement : http://elearnala.wikispaces.com/liveconferencetools