How do you pronounce words like 「ごめんってば」 or 「日本人っぽい」?
You're asking about two archiphonemes:
- The mora nasal phoneme /N/, written
んin Japanese orthography
- The mora obstruent phoneme /Q/, written
っin Japanese orthography
The exact physical pronunciation of these phonemes depends on the phonetic context, in particular on the following sound. Let's look at /Q/ first:
一杯 いっぱい /iQpai/ [ipːːɑi]
一点 いってん /iQteN/ [itːːẽɴː]
一通 いっつう /iQcuH/ [itːːsɯː]
一歳 いっさい /iQsai/ [isːːɑi]
一兆 いっちょう /iQčoH/ [icːːɕoː]
一章 いっしょう /iQšoH/ [iɕːːoː]
一回 いっかい /iQkai/ [ikːːɑi]
The transcriptions in square brackets are phonetic, showing how the sound is physically produced. In each case, you end up lengthening the following consonant. When you pronounce 一通, for example, the [t] sound begins on the second mora and continues through to the beginning of the third mora. The [ː] symbol shows this; in IPA, this symbol is used to indicate a lengthened consonant or vowel.
So when you pronounce /Q/, you aren't simply pausing before you pronounce the next sound. You're actually starting to pronounce the following sound, and holding that sound for an entire mora.
Likewise, the way you physically produce the /N/ sound depends on the following sound:
三アンペア さんアンペア /saNaNpea/ [sɑ̃ɰ̃ːɑ̃mːpea]
三位 さんい /saNi/ [sɑ̃ɰ̃ːi]
三有 さんう /saNu/ [sɑ̃ɰ̃ːɯ]
三円 さんえん /saNeN/ [sɑ̃ɰ̃ːẽɴː]
三億 さんおく /saNoku/ [sɑ̃ɰ̃ːokɯ]
三回 さんかい /saNkai/ [sɑ̃ŋːkɑi]
三号 さんごう /saNgoH/ [sɑ̃ŋːgoː]
三兆 さんちょう /saNčoH/ [sɑ̃ɲːcɕoː]
三通 さんつう /saNcu/ [sɑ̃nːtsɯː]
三等 さんとう /saNtoH/ [sɑ̃nːtoː]
三度 さんど /saNdo/ [sɑ̃nːdo]
三歳 さんさい /saNsai/ [sɑ̃ɰ̃ːsɑi]
三章 さんしょう /saNšoH/ [sɑ̃ɰ̃ːɕoː]
三時 さんじ /saNǰi/ [sɑ̃ɲːɟʑi]
三千 さんぜん /saNzeN/ [sɑ̃nːzẽɴː]
三人 さんにん /saNniN/ [sɑ̃ɲːːĩɴː]
四杯 よんはい /yoNhai/ [jõɰ̃ːhɑi]
三泊 さんぱく /saNpaku/ [sɑ̃mːpɑkɯ]
四百 よんひゃく /yoNhyaku/ [jõɰ̃ːɕjɑkɯ]
四フィート よんフィート /yoNfiHto/ [jõɰ̃ːɸiːto]
三部 さんぶ /saNbu/ [sɑ̃mːbɯ]
三枚 さんまい /saNmai/ [sɑ̃mːːɑi]
三ヤード さんヤード /saNyaHdo/ [sɑ̃ɰ̃ːjɑːdo]
三塁 さんるい /saNrui/ [sɑ̃nːrɯi]
三割 さんわり /saNwari/ [sɑ̃ɰ̃ːɰɑɾi]
This is an example of assimilation, where one sound becomes more like another nearby sound. The reason this happens is physical. Your lips, tongue, and so on are moving into place to pronounce the next sound, and this affects the preceding sound.
For example, when you pronounce コンビニ, the /Nb/ sequence becomes [mːb] because your lips close together in preparation for the following /b/ sound. In technical terms, we can say that /N/ assimilates to the place of articulation of the following sound.
Understanding that, it should be clear what happens with a /NQ/ sequence:
ごめんってば /gomeNQteba/ [gõmẽnːtːːebɑ]
日本人っぽい /nihoNǰiNQpoi/ [nihõɲːɟʑĩmːpːːoi]
In each case, the following sound is a long consonant:
- /Qt/ is [tːː] phonetically, so /NQt/ becomes [nːtːː].
- /Qp/ is [pːː] phonetically, so /NQp/ becomes [mːpːː].
Please note that the rhythm of these sounds is very important! The nasal sound is long here because it takes up an entire mora. The following long consonants are even longer, because they take up an entire mora, plus the first part of the following mora. That's why they're written with the length markers like that.
For more information about Japanese phonetics, I recommend The Sounds of Japanese by Timothy J. Vance. This material is covered in Chapter 5 of his book, and the examples in this answer are mostly taken from his book.
When I speak at normal speed...
「～んって」 - I think I pronounce the ん the same way as in 「んて」(ん+/t/).
「～んっぽい」 - I think I pronounce the ん the same way as in 「んぽい」(ん+/p/). (like /m/ ?)
I think I pronounce んって in ごめんって as 「ん sound in ん+/t/ + gemination + て」, and んっぽい in 日本人っぽい as 「ん sound in ん+/p/ + gemination + ぽい」(which is like "...jimppoi").