Audio problem: When the head of the Federal Sanitarium gives over control of Michael to the Doctors from Smith's Grove, he says, "I can safely say that Michael Myers is in your hands." If you watch his lips, you can see that the sound of the words and his lip movement is way out of sync.
Audio problem: Michael tears Rachel's sweater. The problem is it's not actually being torn, just sounding like it is. (01:19:50)
Answer: It's never explained who told Jamie of her murderous uncle, but Jamie is mocked and made fun of by her classmates for being related to the infamous Michael Myers. She could have been told by her adoptive family, classmates, neighbors, etc, but the movie never provides any definitive answers.