You mean the background Cheer?
What we do see on television or YouTube is an edited video. Ofcourse the sound comes from prerecorded sounds from the people who might be now dead. It's not the audience. They are super less because of the pandemic. So they generally either play the prerecorded sounds or either they add them while the video might be going online etc. Either way its not a big matter of concern.
Those inside the stadium will hear a steady stream of fake crowd noise from the public address system, and those watching on TV will hear a different feed of fake crowd noise on the broadcast.
taken from google
Just like this and at the same.tike it's not just sports matches that use such things , most of the times there are cooking shows, there are funny stand up comedians etc.. all of them use such things.
It would be a funny thing to have a silent show. I would like it who knows how it will go though. There will be one or two scattered whistles, audiences are rarely in tune.