Good music is probably one of the most important parts of a videogame, yet it is one of those things which is often mishandled. I guess I might as well write a few articles about it.
I think the single most important pieces of music in a videogame should be the boss battle themes. This is because boss battles are typically the most dramatic and important places in the game where the player actually has control over the game. Obviously, dramatic moments should have appropriate music to match, but it seems that a lot of games tend to put more effort into making good, unique pieces of music for cut-scenes, rather than gameplay. I believe that doing so is a serious mistake. Videogames are interactive, and the most dramatic and interesting moments should be the gameplay phases of greatest importance to the story (which are usually boss-fights). The music of the game should reflect that.
The worst possible situation is having the same music for every battle in the game, no matter how important the battle. The RPG Xenosaga: Episode 1 actually comes very close to this extreme, since only the final battle has a different song than any other battle. Whether it is a regular battle against grunt soldiers, a desperate battle against alien monsters attacking a civilian population, or a showdown with one of the game's major villains, you hear the same music. Xenosaga had many great songs, and benefited from the talent of the great composer Yasunori Mitsuda, but all of the effort was placed on cut-scenes, as if the gameplay itself was unimportant. Even worse, there are huge stretches of gameplay that are completely lacking any music at all. It hurts the player's experience, and shifts all of the dramatic energy of the game to the cut-scenes, rather than gameplay. I think this is one of the more important reasons that many denounce the game as being more like watching a movie than playing a game, even though it has very long gameplay sequences.
The opposite extreme would be to have a game which has a unique, high-quality song for every gameplay sequence, but that is extremely impractical. There is probably a short action game of some kind which does this, but I don't think I have ever played such a game. It would be a nice experience, though.
I think there are a few rules that should be observed when designing music for action sequences.
1) Music for normal action sequences doesn't need to be too elaborate, but it should never be boring.
2) It is good if the normal action music changes as the game progresses. New stages or new phases of the story should be reflected with new music, especially if there is a large change in the tone of the story.
3) Action sequences which are particularly important to the plot, or are particularly difficult or exciting, should have distinct or even unique music to match.
4) Any battle against a recurring character should have a special piece of music based on the that character's theme music. The same applies if the enemy belongs to a recurring group of characters.
5) Each phase of the final battle should have a unique song.
I should probably write about character theme music sooner or later.