I think it depends on the programme, Anniebach. I hadn't noticed that it happened all the time.
What I do find annoying, and is maybe connected, is that subtitles come on by default, and it's a faff to switch them off - it happens at the beginning of each episode of a series, too.
Yes, they are called 'closed captions' and on my screen the CC button is on the bottom right of the screen. Yours might be different, as I watch via Amazon, but they should be there somewhere.
I don't know about BritBox but I used to have trouble hearing speech on TV programmes. Having now got a digital hearing aid connected directly to the TV I can hear every word! So I presume it was my hearing at fault.
However, even now I occasionally watch a programme or movie where the speech is significantly quieter than the background noises. I had to turn the volume up and down last night on a movie. That was with the hearing aid! So there definitely is sometimes a problem with sound quality which varies with different programmes or movies.
Thank you Chestnut I have no problem with the usual tv channels but the apps are a problem, I think I will have a hearing test if I can have a home test .