Fuck some of you make things 100 times more complex than they need to be
Step 1: What inputs does the 5.1 system have? At a guess, being a PC system, optical, coax digital, 3.5mm and maybe RCA.
Step 2: MySky box has at least optical out, so you would use that.
Worst case, you use RCA out (which I'm 99% sure it has) and get a cable like this to connect to 3.5mm:
http://www.amazon.com/Male-3-5mm-Stereo-Female-Y-Cable/dp/B000I23TTE
Separate audio outputs are going to be with us for a long while yet, just because HDMI is here doesn't mean things are all fucked up.