I've posted earlier about the picture quality of this series, but I felt I had to comment also about the sound.
This is not an in your face, heavy on the subs and surrounds mix, but I am struck by the clarity of the speech. So often in movies - and occasionally TV dramas - I am struggling to understand what an actor is saying, wishing I'd activated the subtitles. But here every word is clear, a tribute to the on-set recording, and also to a well-crafted sound mix. Additionally, I never get the feeling that the entire track has been subjected to a dynamic range compressor, the easy way out for so many mixes (though often the fault of the transmission channel dumbing down an otherwise fine mix). So here's thumbs-up to another aspect of a fine television series.