pronunciation ん＋ [s]

Question

For a few weeks I have been studying the pronunciation of ん and all allophones.

there are many allophones, I was able to learn almost all of them. however there is a case where I have problems ん　＋　[s]

when ん is followed by a vowel, ん becomes a nasal vowel

権威　けんい　　[keĩi];
恋愛　れんあい　[reĩai];
禁煙　きんえん　[kiĩeN];

I have no problem with this. but in the video below it is explained that when ん　 is followed by fricative sounds like [s] and [h], it also becomes a nasal vowel

音声　おんせい 　[oĩse:];
感謝　かんしゃ　[kaĩsha]

https://youtu.be/2Iv-M7tnm6Q?t=181 (3:01) and (3:06)

according to the video, ん of 感謝 and 音声 also becomes a nasal vowel. But I clearly hear [onse:] [kansha] instead of [oĩse:] [kaĩsha]

I have listened audios from different people and I always hear the same thing. In theory the word 先生　is also pronounced [seĩse:] but I always hear [sense:]

I don't know if my ears can't hear nasal vowels in these cases, and Japanese can clearly hear the difference.
Video is wrong and 音声, 感謝,先生 are pronounced [onse:]; [kansha]; [sense:] ?

Thank you very much for the help

Answer 1

When English speakers say words like “once” and “tension”, a little plosion happens before [s] or [ʃ] resulting in sounds some Japanese speakers may hear as ワンツ and テンチョン, respectively. You only need to avoid this when you say words like おんせい and かんしゃ in Japanese. If you can do that by pronouncing ん with the [n] sound, that’s fine. Most Japanese speakers wouldn’t even notice. It’s just easier if the tip of the tongue is not touching the roof of the mouth from the beginning, like [õse:] and [kãɕa], and that’s how most native speakers pronounce it.

This problem doesn’t occur with [h].