The first, of these words, a verb meaning "to encase, enclose, or surround," is pronounced "en-VEL-uhp." The noun envelope, meaning "something that envelops," "an enclosing wrapping," is pronounced "EN-vuh-lohp," "ON-vuh-lohp," or "AHN-vuh-lohp": "Clouds will soon envelop the mountaintop." "Put a stamp on the envelope before mailing it."
Dictionary of problem words and expressions. Harry Shaw. 1975.