Windows is not designed for media production. It's designed for a corporate office. Those people tend to have conferences in terrible acoustic environments because they don't know any better, and they expect their voices to be heard clearly anyway. So Windows has some processing by default to "just make that happen."
Part of that processing is a Noise Suppressor. Similar to the one that OBS has as a filter, except it's in Windows itself. Both of them are designed for spoken voice only. (remember it's for a conference call with terrible acoustics) Anything else is "noise" to be removed, and that includes the lobby music that the clueless conferencer just happened to sit next to the speaker for.
So, you need to dig deep through Windows' settings (not OBS), find anything and everything that is not a straight dumb wire, and turn it all off. Don't just look at the surface that's easy to get to, but go to places where "you're not supposed to be," because some of those clueless business people are also nosy and fiddly, they forget what they did or where it was, and then blame Microsoft for "breaking their computer." So the setting is probably hidden somewhere, and called something that attempts to be somewhat non-technical but ends up being almost entirely useless at describing what it does. But regardless of what it's called, if it's any kind of processing at all, find it and turn it off.
Or you could switch to something that's actually designed for creativity and media production, like Mac ($$$) or Ubuntu Studio Linux (free, with excellent community support, and just as easy now as anything else).
ubuntustudio.org