No idea, certainly wasnt me, its a new tv! Perhaps its a thing they do on secondary HDMI ports, because, as previously mentioned sky works fine with sound through HDMI 1 !
Because Sky+HD doesn't output DD5.1 through HDMI - PCM stereo only. And Sky have set it up like this precisely because many television sets can't handle a digital audio input as they have stereo only processors built into them, so Sky would be inundated with phonecalls from people not getting audio on their telly through the HDMI input either.
In a nutshell - the majority of older TV sets have only a stereo processor. Therefore, they can only handle PCM stereo input. Feed them a digital signal (DTS or DD) and they can't handle it - so produce no audio output. You need to configure the equipment which is feeding this audio to the TV to only send a PCM stereo signal for the TV to be able to process the audio.