What kind of surround are you talking about? Pro Logic, Dolby Digital, DTS, something totally different (SDDS for home maybe?) This will help determine what kind/if you can find circuits for it. Also how much of a "kit" are you looking for? You can always look to buy an evaluation board for the particular chip decoder you're interesetd in. They're usually pre-assembled, but they're not a complete setup. (They lack displays and overly user friendly input controls) As to how to do them, there are sites that will tell you how to decode the different formats. (Except for DTS that is, I haven't seen anything on how to decode it) The problem is that if you go beyond Pro Logic, you'll probably either need a license from to buy the evaluation board or you'll have to get a DSP and code it yourself.
is a link that looked cool. Digital Expansion Bus looked like it could be used for DD and DTS and more.
Then looking about my Aureal SQ2500 sound card I found an interview mention their new product, the SQ3500, which is the SQ2500 with updated drivers and a daughter-board, it could do AC3, prologic, DTS, reverb, and EAX2 etc...
"Actually, an Atmel development platform is significantly less than $100. That will get you several chips, an eval board with in-circuit programming and a RS232 connection. The largest chip has 8k of flash with 512 bytes of EEprom and Ram. This is sufficient for just about anything. It also has a free C compiler which I used with general success."