This problem first occurred with BT Sport UHD live football. BT Tech support only provided temporary respite. I terminated my subscription to the UHD sport channel as a result. I now have the same issue with BT Sport HD and I am on the fringe of terminating BT Sport completely as a result.
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.