My two pence (is that even still a coin?), which I've said before and I'll say again;
There is no one solution. But clearly the biggest missing factor is people getting stuck in to songs and letting go a bit. I'm not sure why so many people seem afraid/embarrassed/disinterested in singing a song, but people clearly do. Why are people more inclined to join in, in some parts of the ground than others? Because others around them are. I'd wager a lot of folk want to get stuck in and all it takes is dropping the inhibition and going for it.
The other thing that I do think is a problem is pre match engineered 'atmosphere', it creates the notion that noise and match day experience will be provided, and all you need to do pay your moneys, sit back and soak it up. Problem is, come kick off, it's all down to the fans.
Maybe if you're a silent observer, get to the next home game and promise yourself to get involved just a tiny bit more. Go on. You know you want to.