Why is sound louder in a room with many people?
It is NOT louder if you are talking about something other than a bunch of people making noise themselves. If you clap your hands and you are the only person in the room it will sound louder to you than if you clap your hands in a room full of people even if those people are being completely silent.
Why do you think this might be?
If you have ever staged a concert which requires very loud music, you will have found that hundreds of bodies just 'soak up' audio power. A 'sound check' is never totally reliable until the audience is actually there.
phinds has already asked: "Why do you think this might be?"
Separate names with a comma.