the sound comes from prerecorded sounds from the people who might be now dead.
oh thats creepy as f , playing and there might be dead people watching and cheering in your game inside that stadium but normal at the same time because they arent the one who does this and it will be plain/less lively if no sounds at all .
They even cover the empty seats with interesting design and pictures.
this is what i noticed when i check the video . i tried to adjust the video to make it clearer and replay it many times because the background looks wierd and found out that they dont move but the crowd was still there .