NiceCast - Mac
At this point, Nicecast is the only piece of streaming software for a Mac that we've found capable of producing live interactive webcasts. Learn lots more and download a free demo at the t Nicecast Homepage at http://www.rogueamoeba.com/nicecast/
"About Nicecast" Video (from MacBreak.com)
How to Webcast and troubleshoot audio problems in Nicecast (by Lee Baber)
-- Here is an additional note about audio settings in Nicecast. This is a blog comment I made to a post concerning this issue.