This is probably because the Zone 2 TV is not able to decode multi-channel audio. TV adverts are broadcast in stereo, but many satellite programs/films are broadcast in 5.1 multi-channel.
If you have HDMI connected to a Zone 2 TV, when setting the Zone 2 Source to any source that is the **different** from the main zone, the ‘raw’ audio and video will be sent into Zone 2, as output by the source. If the selected Zone 2 Source has a multi-channel audio signal (eg. BD player/5.1 satellite signal) the Zone 2 TV must be able to play/decode multi-channel HDMI audio, otherwise, no sound will be heard in this case.
We recommend that the Zone 2 TV is able to play/decode multi-channel HDMI audio. If not, then for multi-channel audio sources, only the picture will be displayed in Zone 2 (unless using the Follow Main feature).