I just watch the highlight mini match of champions league last night. I watch it all 9 match, and a bit confused when hear all sound from supporter on stadium because all look the same.
you will hear a sound drum strings with the same tone, and sound cheers where almost the same when the player dribbling the ball almost in front of the goal.
I thing that champions league company just use that fake supporter sound to make their show to be lively, This can also affect the audience's interest to watch on each match because the company banded with their ads.
They are not fake supporters because if they are you can imagine how much champions League management will spend on by paying all their tickets, however is a very common thing because even in most other leagues I normally see that lovely display by the audience, I think is something they all agreed on doing right outside the stadium as they are coming inside, perhaps when the first role display a certain style with hands others will also follow with another different style making it entertaining to see, so the only thing I believe the company is responsible of is the other activities that normally happens on the field but however I always enjoys those things.