I’ve finally got some sort of surround sound going, but only by clicking a “duplicate front” button in alsamixer. There are still 3 speakers that I can’t hear a peep from!
It wasn’t plain sailing in Windows either! For an age I couldn’t hear anything from the back-speakers but after replacing the 3->4 cable that came with the speakers with an old 3->3 cable plus another, it worked. I’ll look at it later again.
Here’s some links for reference:
