Is ㅂ sometimes pronounced as "M" instead of "P" when it's an ending consonant ?
I understand that when ㅂ is used as an ending consonant it's pronounced as "P", but when listening to someone saying "입니다" I hear it as "M", so why is it pronounced this way, or am I hearing it wrong ?

Question

Answer

Yes, ㅂ is pronounced as ㅁ when followed by ㄴ. This is done for ease of pronunciation. There are also some other sound changes to be aware of, which can be found here: https://en.m.wikibooks.org/wiki/Korean/Advanced_Pronunciation_Rules
Many of the sound changes, while listed, are never or rarely heard due to the unusual letter combinations.

Answer

You're absolutely right. That's how "kamsahamnida" is pronounced, for example! :)

http://forvo.com/word/kamsahamnida/#ko