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It has been only a few years since the first time I heard someone pronounce the word “and” with a “t” on the end. It was someone I know who had spent a lot of time in Washington DC, but is from here in Michigan, I think. Now I hear it all the time on the news among people in the media. Often it seems to be while they think, sort of a pause.
Devoicing a final voiced stop tends to happen in some words regardless of dialect. It’s probably more natural than retaining the voicing.
It’s a firm rule in German, and tends to be heard in parts of America that were settled by Germans. It’s also a firm rule in some American black dialects. As black dialects become more common in the media and politics, it’s not surprising that this feature is spreading in the media and politics.
Another possible explanation is that you are hearing an aspirated d. This sound doesn’t often occur in English.
Typically English t is aspirated, and d is unaspirated. Unaspirated t does occur regularly in English in certain combinations, and you can hear the d-like quality of it if you have a good ear. Try stop (vs. top), stank (vs. tank), and star (vs. tar). Likewise, aspirated d takes on some t-like qualities. Try saying Mad Hatter, hardhat, birdhouse, and say them quickly, fluidly. If you listen carefully, the d in these cases takes on aspiration and comes out sounding quite a bit t-like.
You state the person is trailing off to a pause. They might very well add some aspiration, breathing out heavily as they pause for thought or emphasis, making the final d aspirated and sound as if it were quite t-like to most ears.[edit: added the following] To demonstrate the power of aspiration, try these sets of three words, and decide if the t of the middle word sounds more like the sound of the left-hand word or the sound of the right-hand word.
tab; stab; dab
tuck; stuck; duck
teal; steal; deal
tick; stick; dick
tock; stock; dock
I was just thinking about the word “asked” which is always pronounced with a “t” on the end, but with no start of a “d” before it. This is a clear case of devoicing. But “and” is being pronounced with a “d” before the “t”, and I think aspiration might be the best explanation. And I never heard it before a few years ago.