Some people are just too simplistic about this.
This is what happens at City.
That stupid PA shuts up and the game kicks off.
The South Stand start a rendition of "We are not really here". 111 start a rendition of "Bluemoon" at exactly the same time. the rest of the ground sit in silence.
the away fans start up a song. The south stand start up a chorus of "If you hate Man United". 111 can't hear them because of the away fans so start off something else. The South Stand's song doesn't get going beyond those 2 blocks because the Colin Bell stand never sings.
Eventually, after 5 minutes of this, the rest of the ground feel isolated from the signing and sit in silence, the two singing areas start to get annoyed that their songs are not catching on.
An important point to note is that when songs do get sung right round the ground, in my opinion they tend to start off at the other end - the North Stand! This is because it is one of the few occasions when both these two sectins hear the song and sing together.
With one voice, people like me who sit up the side but would get behind the team with a song if it spread, would sing much more. As it is, I'm not joining in with a muddy noise from two ends.