We talk alot about "tah" and "dah, "toh" and "doh", and they are supposedly important as far as brass playing concepts go. However, my wife (a speech pathologist) told me the only difference between the T and D sounds is vocalization.
Meaning, with a "D" sound the vocal cords begin to vibrate before our tongue is released, whereas a "T" syllable requires the release of the tongue before the vocal cords vibrate. The tongue position is the same, in theory, for both.
Just something to think about.