The answer to the question posed in the headline is a simple one: the Guild Wars 2 sound team. It's how they do it that's a little tricky.
I'm not going to say that creating the sounds for a regular game isn't a daunting task. It's just that normal console and PC games have a finite amount of content. Crafting the sound for a massively multiplayer online game involves making sure hundreds of thousands of players are hearing the right things at the best quality at the right time. Every footstep, every creaking floorboard, and every rustling leaf needs some sort of sound treatment. When you immerse yourself in the world of Guild Wars 2, these guys will be your ears. You gotta respect that.
Might I add that it is my dream to one day be featured in a video with a banjo soundtrack? Thanks for stealing my dream, sound team.