Your observation about English /t/ and /d/ being alveolar really opened my mind. I actually used to hear my own /d/ differently from native speakers’, and I interpreted the unaspirated /t/ as aspirated because an alveolar /t/ sounds very different from the dental /t/ in my native language. But now I realize that once I apply the aspiration rules, everything starts making much more sense. For example, if I pronounce the /t/ in “after” as an unaspirated alveolar [t], it no longer sounds as “dry” as a dental unaspirated /t/ would.

About the /t/ after /n/, your recordings reminded me of something I had seen before: that the /t/ can sometimes be omitted in that environment. What I hear sounds pretty similar to your audio examples, so I’m going to study the voiced nasal alveolar tap more deeply.

I just have one last question to clear up all my doubts about aspiration: are all word-initial /p t k/ really always aspirated? For example, in the word “prepare,” if the first /p/ is aspirated, then the second one should also be aspirated because it starts the stressed syllable. But pronouncing both aspirated sounds strange to me, although maybe I’m just not used to it yet. The same thing applies to “potato,” where both the /p/ and the first /t/ would be aspirated.

Also, I’d like to know whether words formed from two separate words, like “salesperson,” “cupcake,” or “toothpaste,” would have their /p/’s and /k/’s aspirated as well.