There are five segments in the clip. Are all the ts in "But I..." pronounced out the flap t sound? I personally can hear the flap t sound in the second and third segments. The first and fourth are not that clear. And the fifth, I don't hear the flap t sound at all and the t sounds like a stop t or a silent t. Are there any native English speakers help me with that?
TLDR: all of them are flaps except for the last one which sounds like an unaspirated t.
First and foremost, native speakers think of all the T's a single sound—/t/, that is, all those sounds are stored as a single sound ('phoneme') in their minds so they might not notice the phonetic difference between a flap and a normal t. A phoneme is a 'meaningful unit' in that it distinguishes one word from another, for example, tap is distinguished from cap by a single phoneme (/t/), so we would say that /t/ and /k/ are different phonemes because cap and tap are different words having different meanings. I recently came across a language known as 'Samoan' where [t] and [k] don't distinguish words i.e. they are a single phoneme (meaningful unit) so we would say that both [t] and [k] are the allophones of a single phoneme /t/ (or perhaps /k/, I'm not entirely sure).
Likewise, [tʰ], [t], [ɾ], [t̚] and [ʔ] occur as allophones of the phoneme /t/ in English. Although native speakers do (and can) hear the difference between all those sounds, they think of them as the same sound—/t/. Unfortunately, native speakers cannot help you 'hear' the flap, the only thing you need is long-term exposure to hearing the flap. In the clip you provided, all of them are flaps except for the last one which sounds (to me) like an unaspirated t: [t˭].
- [tʰ] is called aspirated t which occurs at the start of a stressed syllabe as in time, top, tool etc.
- [t] is a t sound without aspiration. Unaspirated t usually occurs after an S as in stop, steel, stall etc.
- [ɾ] is called an alveolar flap which usually occurs where an intervocalic (between vowels) /t/ is unstressed. It's mostly found in American accents.
- [t̚] is called unreleased t i.e. no released of the closure made for the t. It usually occurs at the end of an utterance (as in bat in some accents)
- [ʔ] is called glottal stop. It usually occurs at the end of words as in hate, mate, but in some accents.