But there is also another type of applications which fall in between the two of the above types. They actually act as the "middle-man" between those two. Although sometimes they come separately but usually they're attached to user interacting GUIs, most of the time.
In that sense a sound server is actually that "middle-man" which sends/receives audio related information between the actual OS hardware level software (the Kernel, in GNU/Linux OS) and your sound card speakers, etc which "carries" the "mixed" sound-output.
Unlike with the original sound subsystems which were not that easy to configure since they were "close" to the Kernel or the PC hardware, thus even a small fluctuation could jeopardise things easily. So as a fix, later came the sound servers which run in a more "close to users" such as above mentioned applications thus they're easy to configure + even if a sound server fails the stability of the OS is not affected that much since it's doesn't run closer to the hardware level or at the core of the OS.
These days we have few main sound servers OSS, ALSA, ESD, etc and the latest is the Pulse Audio. Although personally I don't like it that much since it doesn't work that well with my sound card hardware yet many prefer it since it does come with many features and better compability (not in my case :D) nonetheless.
So if you use Ubuntu 11.04 Natty Narwhal and want to configure your sound sever related setting such as ...
*. Local network access to your sound card.
*. Configure various audio-network-server related setting such as - enable/disable the access, automatic authenticate, enable/disable DLNA or UPnP media server access, etc.
*. Configure audio multicast severer settings (RTP receiving and sending) - send audio from local speakers/microphone or create a new virtual device for the RTP multicast.
*. Add simultaneous output support.
That's actually about it. It's a small application, a font-end actually, written in GTK+ toolkit and has a tabbed interface and very easy to use + can be extremely useful as well.
You can install paprefs (pulseaudio preference, tool) in Ubuntu 11.04 Natty Narwhal by entering the below command in your GNU/Linux Terminal.
sudo apt-get install paprefs
You should be able to access it via: "System" -> "Preference" -> "PulseAudio Preference" or simply type "Alt" + "F2" keys and enter "paprefs" in the run box. That's it. Good luck.