Hmmm, if the sound on the sky box is on mute then it should not come through your HIFI's speakers unless you have connected the sound so it will play through the HIFI speakers. If you haven't then I don't know why this is happening.....
Unless, maybe if you go into settings > volume >> ensure its muted that way but I don't know if
a- you can do such thing
b- wether its actually worth all the hassel