I'm not an expert on iplayer but suspect it may be something to do with the actual sound track format. I'm sure somebody on this forum will know what the different formats are.
Does the sound get through the AV Rx for both StdDef and HiDef TV, and similarly for recorded shows on each.
HiDef uses Dolby Digital Sound, StdDef doesn't for instance.
At one point the YouView box didn't output DD 5.1 (downgraded it to stereo) but I think they have updated it now.
I suspect there is a similar difference on iplayer.
I think you were talking about passthru to TV, but Does the AV Rx output the sound through its own speakers?
Some AVs do not intercept the HDMI sound, you need a separate digital feed (optical, coax).
You could experiment a bit in this area to see if the AV can handle the iplayer sound format at all.
Just a few ideas, it may help you narrow down the problem, but it doesn't appear to be the Humax itself, or the TV, from what you have said so far.
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