There may be but I've never heard of anyone doing it. The big thing again with this is space and then heat. Having a dedicated amp housed with pc components would produce an ungodly amount of heat. You would need high CFM fans. They produce alot of unwanted noise. You may get away with a liquid cooling solution but again space would limit you with that. Most build an HTPC and connect to a dedicated Amplifier. I'll see if I can find anyone trying what you are wanting to do. Who knows? I'm sure there's someone who has rigged up something.
OK. I haven't found one person/company offering 5.1 amplifers combined with a pc within one case. What I did think of though may meet your need depending on your taste in high fidelity components. If you are looking for a decent sounding home theater system and your not planning on using it in a large space then I would recommend looking at amplified speaker systems. Here is a review of a 5.1 wireless(rear) THX certified system from Logitech that had great reviews. http://www.pcmag.com/article2/0,2817,1904834,00.asp
Other than that I think that you are going to have to have a seperate dedicated amplifer to run you speakers.