I have the BT 4K box.
My TV is hooked up directly to my Smart 2 Hub via Ethernet cable. Hub speed is consistently within the 71Mb download expected.
I experience no sound issues via any other streamed services, ie, Netflix, Disney and Prime. All other TV channels are via Sky, including live sport.
Sound output is via a Sonos Arc which is connected to the HDMI Arc output from the TV.
Help please .....!
Mike