Being on the stadium, you might hear different sounds but it all aggregates to a different large noise to viewers. It's mostly cheering that's heard when their are goals. I don't think they add any fake sound effects as that would sound unnatural.
About the viewers on ground, the demand for tickets of champions League matches high and if both the team participating don't have a huge fan following, they'll just lower the ticket more people would come to the game. They may even provide free passes to be a part of it. During our school, we used to get free ticket to support a random team. UEFA might employ those tricks.
Yes, it could be that the sound inside the stadium during the match will be heard as one so that the cheers of the audience in the stadium can be heard very loudly.
Yes, the audience could get free tickets but maybe only some of the audience and maybe at that time you were part of the lucky audience.
Yes, it could be that UEFA will issue a trick to enliven the match that it will hold so that the stadium is crowded with spectators, one of which is by giving free tickets.