Backround music was done by Sonic Mayhem.
The action takes place in a science-fiction environment. In single player, the player is a human soldier taking part at an invasion of the extraterrestrial Stroggos civilisation.
The latest version is 3.20. This update includes numerous bugfixes and new maps designed for multiplayer deathmatch. There version 3.21 available on Id Software's FTP has no improved functionality over version 3.20, it's simply a slight repackaging to make compiling for Linux easier.
The server code of Quake II, which defines how everything behaves in the game, can be changed in any way because id published the source code of their own implementation and Quake II uses the shared library functionality of the operating system to load the server program at run-time.
Since the release of the Quake II source code, several 3rd party update projects to the game engine have begun; the most prominent of these is known as Quake II Max, followed in popularity--if not improved features--by Quake II Evolved. Generally, such source upgrades improve things like the maximum resolution the game may be run at, the quality of the lighting engine, adding things like shaders or decals. Just to generally improve the look of the game and enhance its playable lifetime.