Isn't technology great?
I have a vision of the future.
It's a hot summer night in (a place near you).
Everyone who wants to see and/or be seen is entering that new great night club.
After paying the price of a cinema ticket (it's a posh place, no riff-raff) each attendee is handed some Google glasses, earphones and a little box to drive them, with a neck strap or belt attachment, depending on attire.
With the glasses and earphones on, attendees enjoy an augmented reality experience.
Those black 16 foot by 9 foot rectangles that looked really odd now show videos thanks to those Google glasses, accompanied by the music being pumped through the earphones.
A hand gesture conjures a mid-air menu.
One of the options is "I'm feeling friendly".
Now your view of the people around you becomes augmented with talk balloons like you see in comics when people are talking.
In the balloon you see various statistics from which you can determine if they would be nice to talk to, as well as their status.
Now to the subject of the post.
The music you're listening to is from a YouTube video, and that video on the wall is from the same video.
The fact that everyone around you is dancing to the same beat might lead you to suspect that they can see the same video and hear the same music, but that's just coincidence.
If it were true then this would be a public viewing, wouldn't it?