I think this basically goes back to the thing I was saying elsewhere about post-aspiration (the little extra puff of breath) on p/t/c sounds in English and Welsh.
We’ve got three different sounds here (broadly speaking – I’m sure you could do a more detailed/nuanced analysis):
- T/D Voiceless, Post-aspirated = English/Welsh initial t, Icelandic t, doesn’t exist in Italian
- T/D Voiceless, Un-post-aspirated = English t in ‘stop’, Icelandic d, Italian t; doesn’t exist on its own in English
- T/D Voiced, Un-post-aspirated = English/Welsh/Italian d
It sounded to me on the recording of @catrinlliarjones saying ‘beirniad’ as though the final -d wasn’t really voiced – especially the first time she said it. Now that might or might not be an artefact of the recording/playback, but I’m sure it’s something Catrin wasn’t aware of doing; and because it still lacks the post-aspiration of English/Welsh stand-alone t (probably what @Novem heard as t sounding like ts), which is distinctive, it still sounds more like a d to us. However, it’s going to make it sound like t to someone used to the sounds of Italian.
In this respect English and Welsh seem to pattern more with Icelandic, where it’s the aspiration that makes the important difference, than with Italian and most other languages, where it’s the voice – notice that Icelandic t is like English t, while Icelandic d is basically the same as Italian t.
…But I really don’t know about the different d sounds in different Welsh accents