On recordings, tried 3 different programmes, 3 different channels. HD & SD. All the same issue. Switched audio on the Humax menu from Multi to Stereo. Same issue.
What channels were the recordings from?
Any advertisement breaks on the recordings or live TV that the sound drop outs occurred? (i.e. are BBC channels/recordings affected the same as CH4/CH5/ITV channels )
Do you have humax remote button "wide" set on Auto/Zoom or Pillar Box?
However, I've found that by ignoring the 'pop up' which comes on the LG's screen as a prompt to switch to the selected HDMI input, and allowing the Humax to fully boot up before switching the input, the drop outs are reduced considerably.
It looks like an LG TV rather than a Humax problem.
Regarding 'Pop Up' messages
On my Vierra when I play back recordings from the humax on CH4 recordings at add breaks there is an annoying pop up in the left hand saying "Auto -16:9" ,and after the interval another 'popup' "Auto-Just" appears. This only occurs when the humax remote [wide] button is set to "Auto". When set to "Pillar Box" or "Zoom" there is no pop up but it seems there is a longer pause/black screen.
The same does not happen on recordings played back from one E4 recording I tried. I haven't tried other channels.
Just a wild chance the sound dropout relates to how the TV switches it's screen display and how the original recording was formatted. Might be worth changing the [WIDE] setting on the humax remote.
[Edit] note I could not reproduce the 'popup' messages ["Auto -16:9" "Auto just"] on my non SMART TV using the same recordings/settings.