Yo! Here's the setup and gizmo you need:
http://www.logitech.com/index.cfm/pr...ONTENTID=10036
The leads on the left (red/white) in the pic are one end of any standard issue adapter cords, the other end of which is a 3.5 mm plug that will go into your old 2 ch (portable or whatever) sound system's output (marked with a green ring, or "Speaker/line out"). Note: this is no 5.1 nohow, just a "misuse" of it as a generic sound amp.
Should work the same with any 5.1 system.