Thank you so much for your help ^^

I think it's because we don't put a stress on "미" usually. As you know, the Korean language doesn't have dynamic tones or accents. However, we use a high tone or accent some times depending on the situation. When we should say "미안해", it's an unexpected situation. In most cases, we put a stress in the end or middle of the word. So ㅁ of 미 may not be heard clearly as like when you pronounce m in "Mother".

It's 미안해 with an M sound. But to my ears, it often sounds like B too. It seems to be somewhere in between. This is why you should learn Korean by Hangul and not Romanization. Romanization is not exact, it can very, etc. Learn Hangul if you can.