Recently, I've been on a "free trial" spree; playing the Age of Conan, World of Warcraft, Everquest II, and Guild Wars week-long trials simply to see how each game is. During this freeloader epidemic, I've noticed the wide variety of instrumental music created and used in each game. I've also noticed how the same music affects my playing experience, as well as my mood about certain activities (as the last thing you want is to be annoyed by horrible music while doing one of your least-favorite activites, such as farming).
Some of the best music orchestra music I have ever heard in an MMO belongs to Everquest II. The music, written by Emmy award-winning composer Laura Karpman and performed by the FILMharmonic Orchestra Prague, is designed entirely with that zone's "personality" in mind. For example, listen to this and imagine walking through a dark, evil, vermin-filled city full of mystery, or listen to this and imagine walking through a forest filled with tall, thick, ancient trees supporting a small colony of nature-loving, beautiful fairies. I'm not sure about you, dear gamer, but simply listening to the music makes me want to play the game, and that is exactly how in-game music should be.
Music is a very important part in an MMO player's experience in the game, and it is a true shame that a lot of mainstream companies don't realize its value or commit more time and effort towards a quality musical masterpiece. Whether you like alternative or heavy metal, rap or techno, a good music selection will always enlighten your day of playing in Azeroth...or Norrath, or Tamriel, or wherever you are.