Sound is as much of a key element to game design as the visuals and mechanics are. Without one or the other, yes a game could still work but, the immersion and sensual link towards the game is broken. For me, good sound design is when I don’t notice the sound directly but when it heightens the experience without taking away from it.
It’s not just adding sound that is important; it’s how you use it. The building of tension, dramatisation of events, etc. are all climaxed by the use of sound and it’s the change in audio that builds to events and differentiates them from what was previous. For example the use of a full orchestra for the battle music gives the sense of grandeur that isn’t felt compared to the other game audio.
Personally I find that good music and sound design resonates outside of its original boundaries just as much as it does when in the game. One of my favourite albums is that of the ‘London Philharmonic Orchestra’ playing music from video games. Even though the music wasn’t intended to be heard in this way then it was written it shows how well it was designed by the fact it still works as music on its own when played by the orchestra. I don’t know whether it’s the nostalgia from some of the tracks but I think it’s just the simplicity of the music that echo’s and really makes it work when played at such a large scale. For many of the pieces it was down to the limitations of the original platform that brought this, but hopefully with the new technology coming and the increase in recognition sound is getting as its importance to games we will start to see more great music and sound coming out of the industry.