Claud,
I'll have to disagree with you on this...the pronunciation in question is in the Liber Usualis.
Okay, but as a big primary-source guy, I'm wondering where the Liber came up with it. It is not a historical pronunciation in either Classical or Ecclesiastical Latin that I know of, although ELS's quote is very interesting in that regard. The question is..was "michi" a provincialism, or an authentic pronunciation of the classical period?
This is making me want to dig into my Italic historical linguistics again.
So I ain't sure what to make of this. Alls I know is, durnit, I ain't sayin' that dang k. ;)