I usually say,

EN-velope: the noun
en-VEL-up: the verb

but my mother says "AHN-velope" for the noun. She also says "b-AH-th" (although she'll say "bathroom", as in "cat".)

Also, I work with a Canadian woman who says "EN-velope" for the verb, too. Is that a Canadian English-ism, Bean?