There's two ways to go with 5.1 audio. Most headunits that feature this will be designed for DVD (or Bluray), and will read the discreet channels from the disk. The other way is to read the stereo signal from anything (mp3, cd, etc) and create
the 5.1 from that. This is what Dolby does with their Prologic series of software/hardware (there's other ways to do it, but this is probably the most popular). I can't think of any headunit that does such a thing on it's own. There's external processors that will do it, in fact IAmSonic has one for sale in the "headunits" section of this site. There's several ways to create your own imitation 5.1, usually with a L+R signal for the center channel and L-R mono for the rear surrounds.