PulseAudio is a cross-platform, open source sound server that supports advanced software mixing capabilities and network transparency. PulseAudio is designed to be a drop-in replacement for the Esound daemon and uses a pluggable Esound module for backwards compatibility.
PulseAudio provides the infrastructure needed to bring a next-generation audio experience to the Linux desktop. PulseAudio will make it possible to independently control the volume of individual applications, support moving audio streams between devices on the fly, output audio on multiple devices simultaneously, and provide compatibility with GNOME's fast user switching functionality.
http://arstechnica.com/journals/linux.ars/2007/10/17/pulseaudio-to-bring-earcandy-to-linux