People do frequently pronounce "might've" with a soft "v" sound, to the point where some (not well-educated...) people think the phrase is actually "might of" instead of "might have." Don't make this mistake when writing!! "Pieces" should always have an ending z sound. If you pronounce "caused" with a "t" sound at the end, people will think you're saying "cost."

No, none of that is right. You'll sound as if you can't speak English properly. The " 've " has to be a "v" sound, not an "f". The "d" at the end of "caused" must be heavy. If you use a "t" sound, I hear a funny pronunciation of "cost". For "pieces", it ends in a "z" sound. If you want to record your voice and post a link here for feedback, then vocaroo.com is a fairly useful site for that.

It is not uncommon to hear some people pronounce 'might've' as 'might f' but this bad grammar. The correct pronounciation is 'might have'. However, if you wish to shorten it by using a colloquialism, you would pronounce it as 'what might' f cos'd it'. You are correct with 'pieces' you should pronounce it as 's' and not 'z'. Good luck.