The stadium was designed to ensure that sound didn't escape. I remember the objectors at the Public Inquiry making a massive thing about how intrusive the sound of a 20,000 crowd would be if the Club didn't take active steps to prevent the noise escaping.
I was in the North Stand and was completely unaware of two sets of singers "competing against each other's songs". As has been pointed out, the sound of the singing in the North Stand doesn't travel. What I did hear sounded loud, but I guess that's how it is when you are in the midst of a good...