The big most expensive TV in the living room has the worse sound.
Wife bought a cheapo $65 Walmart sound bar it is junk but it sounds better than the TV speakers.
Both of those things are because of bass frequency response. Clearly you do better with sound sources that have the bass "turned down" either mechanically (smaller speakers) or electronically. That is because, in *very* round numbers, the lower frequencies are the vowels and the higher frequencies are the consonants, and it is the consonants that carry the meaning. Acoustically, the differences among B, C, D, and T are higher-frequency effects, harmonics, etc. Vocal chords are doing the exact same thing for all four.
BTW, smaller speakers are not automatically better for you, especially for products like sound bars that have internal amplifiers tuned to compensate for the deficiencies of small, cheap speakers.
For the set with different preset voices, hunt for the config screen with straight treble, midrange, and base adjusters. This might be on the preset menu under "other", or something like that. Boost the midrange a little, cut the bass a little, and re-evaluate.
Of course, another cause is that the dialog in some British programmes is just plain hard to decipher. I love Jodi Whittaker as Doctor Who, but some of her more rapid speech might as well be chinese.
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