Can I ask what region you live in? Pronouncing <womb> as /wuːm/ is the dominant pronunciation in every English speaking country, so much so that I can't recall ever hearing it differently, and I'm surprised you've never heard it. It's not a recent change either, the /b/ disappeared somewhere before the 14th century.

New England. I've never paid enough attention when traveling to notice (those words aren't exactly the most common to hear), but when I say "woom" my lips end closed, and when I say "womb" my lips end opened after closing. The b isn't "voiced", per se, but it does change the way the word sounds to me.

And to be fair, I am aware New England English is its own distinct thing so I might have grown up in a bubble:

https://en.wikipedia.org/wiki/New_England_English

NE Ohio and I always pronounced the b too (akin to New England I guess)