If you click on the "x" on a open tab, it will most likely close it. If you click on the "x" on your browser, it will close the browser, maybe will it alert you that you're going to close many things.
Open Microsoft Excel or Microsoft Word, Notepad, your Windows file explorer, same thing.
Those are probably the most used app in the world, so people will assume that it is what the "x" button does.
But if I click X on my email client, it keeps receiving email; if I click X on my torrent app or JDownloader, they keep downloading; if I click X on Steam, it keeps updating my games; and if I click X on a chat application, it keeps receiving messages and, yes, usually keeps voice chat open.
And BTW, browser tabs sadly do stay open in a twisted way: notifications, often malicious for the kind of users that accept the prompts without reading.
It is the wanted behaviour, of course, but I don't find it weird that having a message explaining it the first time you close the window should be mandatory.