There are a lot of 'don'ts' when it comes to video game music. The most obvious one is repetitiveness. Back when the option of turning the music off didnt exist, I wouldn't have wanted to play a game with no sound what-so-ever, just because the music got on my nerves.
Just like games have progressed visually, so has the audio, and the use of composers is becoming standard now they realise just how much impact it can have.
Simplified, there are two kinds of sound in a game. The underlying theme music and the interactive sounds. Unfortunately, I have overlooked music unless it was quite noticeable or rememberable.
Games like Halo and Gears of War have produced full length musical scores as an undertone to gameplay, with the music used to create feeling within the player at certain places in the game. I feel that Mass Effect had a particularly powerful ending in that respect, as the music was used to heighten tension and create a feeling of urgency.
One game I remember and appreciate for the environmental sounds is Half-life 2. Subtlety is used to their advantage and it completely works. The game is a vast landscape of realistic noises that sound original just because they dare to keep it simple.
Sounds alert us to the world around us and what's going on. This should also apply in game. How do we know an enemy is there? You could have seen them, but you probably heard them first. When enemies have distinct sounds, it telling you how you should react, what weapon to use, whether to fight or flee etc.
Of course sound shouldn't be so loud and contrasting that it's competing for your attention and confusing your reactions. It can be a powerful tool but developers need to know when to create an audio feast and when to tone it down a notch.