Hi,

If you search on Amazon, there are a number of HDMI audio extractors. These will sit in-line of the HDMI cable and present digital audio on an optical output or 3.5mm analogue jack.
They effectively pass through the HDMI video signals and split out the audio.

They say that they work with 4K signals - so should not be a problem, even with the Sky stream box...
They're about £20, so not expensive.

May be worth giving one a try.

Cheers,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.