How many of these do you plan to post? I just don't want to clutter up the music board, so if you're gonna post too many (more than 4) I would think it'd be best to put them in the same thread (I could do that for you per request)
So this is my opinion.
Well if everyone was into metal, that doesn't exclude people enjoying any other music genre but metal would be the most commercially successful music genre, it would also be a universal, with cultures who haven't even discovered electricity music still being a fan of metal, causing great confusion and increased funding to study in aesthetics and human psychology with interaction of music. Most popular music would be based on metal and metal wouldn't be called metal, but probably pop.
Now answering what would happen if everything is based on metal is difficult, so I'm just going to assume you mean if everything was based on what is currently perceived as, in general, metal represents. In which case, there would be universal cultural stagnation as everyone is believing in the same lifestyle, such stagnation could actually RESOLVE all conflict of the world, but more likely would cause conflict on trivial issues within metal. If everything was based on metal, countries would actually go to war on issues that could be as ridiculous as "this country said we're not as metal as the other country" or even what singer is the best, musical warfare would also be much more used. Societies would be considerably more warrior based, with the strongest warrior being considered the highest of authority, such unstable societies would collapse with large numbers and tribal warfare would ensure OR that akin to roman a empire. Scientific advances would be low, to almost none, as the society would reward those who are strong, not those who are intelligent, this would lead to a very prolonged scientific dark age. Religion would be roughly unchanged, as religious beliefs would be plentiful (and they're plentiful today) although religions would drastically change, to something more akin to Norse mythology than Christianity. Also most importantly, musicians would be the highest person in society, often being leaders and considered "the best warriors".
This is all speculation.