[MPlayer-users] intro to audio?

[Brian Craft]

Can anyone point me to info about audio? I haven't followed audio for a few
decades so I need the basics: 

What is AC3 and 5.1? 
What formats are used on DVDs?
What hardware is needed to playback the relevant formats on linux?
Are there options wrt cpu decoding vs. digital pass-through?
etc. 
Are there any howto's for doing dvd audio with linux?

Also, are there any easy solutions for programatically selecting audio sources?
I currently have a video grabber board (stereo line out), ancient stereo sound
board, and a vcr. I use all of them with the linux box, but haven't found a
good way to control which source is active.

b.c.