Ahnvelope people always seem to be the same ones who say miss-chee-vee-us for mischievous. Of course, that pronunciation of mischievous is just wrong according to all dictionaries.
Maybe that’s why I always pronounce ‘Caliope’ as Kally-ope.
There are several “heat maps” that depict pronunciation and vernacular across the US (seriously, in parts of the south, rain and sun together is “the devil is beating his wife” ?) but there does not seem to be one for “envelope” (nor one for “skyedjewel”).
N-velope, not On-velope. Born, raised, educated in central Midwest, where we do it right. Eat your 
out.
EN velope. I would not say ON velope. If the mountain tops were enveloped by clouds, I would not say that they are onveloped by clouds. It is the same meaning for a slightly different word; to wrap, cover, or surround. That is what an envelope does, that is what the clouds are doing to the mountain tops.
Onvelope is just too pretentious. And wrong.
But envelop is a verb. That changes the stressed syllable and affects the pronunciation of the first syllable, as I posted earlier. How do you pronounce encore, entree, entourage, entrepreneur, ensemble? And do you say UH-tribute instead of AT-tribute because you uh-TRIB-bute the same pronunciation to both noun and verb forms?
Saying EN-velope isn’t wrong, it’s just different. Same for AHN-velope. Neither is more correct. But using the pronunciation of envelop to try to prove something doesn’t work.
I think I say it both ways, perhaps depending on my audience? There may be times I don’t want to sound too posh.
En-VEL-o-pee.
Catchy, isn’t it? 
In which you can place an invitation to Penelope, to go out on a date and listen to a calliope. 
Not bad-now make it into a limerick.
I’m in western Canada of Swedish/Scottish ancestry and say “ENvolope” but those of Ukrainian ancestry commonly say “AHNvelope”.
Another one is “live”:
“I live well” - short i
“He’s a live wire” - long i
And my favourite, where capitalising a word changes its pronunciation:
“Please polish the silverware.”
" The Polish ambassador presented her credentials. "
Chicago here. I believe the “EN” pronunciation is the one I use most naturally. But the “ON” one also sounds natural to me.
It’s kind of like “coupon.” Sometimes I will insert that “y” there and say “KYU-pon,” sometimes I will drop it and say “COO-pon.”
It depends on what state I’m in. So yeah, I code switch.
EN. I’ve also heard amongst older folks back in the day the pronunciation "EN-vellup,’ with the consonant ‘n’ slightly extended.
Are you vacationing away from your husband, The Dean?
Hard ‘On’ is how I give it.
Maybe it’s like people who call their dad’s sister their “ont” vs their “ant”.
I say “Enn-velope”, but I do think that “Onn-velope” sounds classier.
Is it a French word like Encore?
It appears so.
Hard onvelope?
In parts of southern and southwestern Virginia, it’s pronounced “aboat”.
Me: ENvelope (and for the verb: envELLup"). Raised in central PA, but largely got rid of the Central PA accent (e.g. our nation’s capital is not Warshington) due to chorus, studying several foreign languages, and living in NC for 12 years.
Related question: Is your father’s sister your “ant” or your “ahnt”?
FWIW: “ahnvelope” just sounds like a variation (and yeah, I know it’s closer to the original French word). “ahnt” has always sounded a bit jarring even if it’s a lot closer to how it’s spelled.