On the Soundtouch 300 you have 3 ways to pass multi channel sound directly.
Hdmi in port (not enabled by default),
Use the HDMI/Arc connection depends on your TVs support of that feature, optical can be used if HDMI/arc does not work.
The HDMI in port should be can support 7.1 sources, unlike the optical or HDMI/arc.
if you have a Blu Ray player or. Games Console that has Blu Ray support (especially with a passthrough) that is a good choice for connecting to the HDMI in, or you can use a HDMI switch. Your Sky box should be suitable with all connection methods.
To enable the HDMI in port, access the settings in the Soundtouch app as it cannot be done from the remote.