freshmeat.net: Project details for Walkietalkie
“Walkietalkie is a lightweight application for voice-over-IP (UDP) communications between PCs and Macs. It is low bandwidth and low latency, and ideal for voice chat during netgames. It doesn’t require any servers or other infrastructure.“
Uses a single UDP to link 2 PCs for audio. There is a server available for conferencing. Mac and Win32.