When the sheriff mentions the term "onion snow" to the doctor, he is referring to a term particular to the state of Pennsylvania originated by the Pennsylvania Dutch culture and language, and refers to a snowfall that occurs after the spring onions have been planted, and comes right as they are sprouting.
The song that starts to play once Dr. Carl Winters begins slicing through the corpse for the autopsy is from 1949, it's called "Similau (See-Me-Lo)" and it's originally by Peggy Lee. It has been covered by many different artists.