"The WH sound"

Hello, everyone.

I've been studying the English language for so long and now I'm into linguists.

I've been watching lots of closed captioned TV series these days and there's something strange I've been realising.

The WH sound sounds very strange to me.

Sometimes when people talk very fast, I can't hear the wh sound in some words, mainly in WITH and WE'RE or it sounds very strange. Can somebody explain why???

And what happens to the pronunciation of the contraction "DIDN'T", because sometimes I hear it like it's pronnounced "int".?

Thanks for any help.

Real Cool,