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What's probably happening (it's hard to know given the scarce details in your question) is that the TV is telling the AVR that it should switch to the TV's Audio Return Channel over HDMI. This happens when the TV receives a direct signal that it believes has bypassed the AVR, meaning that you won't hear anything if the TV doesn't pass the audio to the AVR.
Since you're complaining about this, I assume that your installation is not such that SKY bypasses the AVR, but unless you tell us something about the connection topology and AVR setup, it's going to be very hard for us to determine why this is happening or how you can sort it out.