For SKY+ you need a fibre link like this http://www.tvcables.co.uk/cgi-bin/tvcables/TS001.html
Make sure in the SKY box settings you have done the following
For video ouput select RGB not PAL and 16:9 not 4:3
For sound via Fibre set to digital output.
Yes you will get 5.1 from Sky Movie channels or Stereo from all overs, but you can tell your AV amp to switch to Dolby Pro-logic II to get all speakers going. If the WII only has two phono plugs then use a stereo input on the AV amp and again switch to Dolby Pro-logic II to get all speakers going.