the no 5.1 or 4.1, is completely up to your HU.
There is an entire thread lable something like "Can a CarPC compete with a HU sound quality" or something, and again it all has to do with your HU, your computer, and your wiring.
Red has won national SQ competitions with a PC. Others have ground loops. All install and component dependant, it cannot be generalized.
And you can encode anything you want into xvids. You choose that when you encode the video.
If you are talking about pirated films, then 1 that is a no-no on these forums, and 2 usually they are highly compressed so 2 channel sound. You wont benefit at all from anything more than 2 channels.