One of the questions I've asked The Saboteur team is whether or not the visual changes of their "Will to Fight" system will be mirrored by changes in audio. Locations heavily occupied by Nazis will be mostly black-and-white, and will be colored more as the Nazi occupation is lifted. I figure this could be accompanied by audio effects, like making the music scratchy (like old records) or distant (reverb, gating) in the black-and-white settings.
Anyway, that general concept of connecting visual events with audio filters can be applicable to many games.
For example, most people have experienced static in radio signals while driving. They know this is due to distance from radio towers or interference. So degrees of static can be used to give the player an impression of entering wilderness, being cut off from civilization, or to suggest looming events... like an alien invasion!
Reverb effects can be connected to events for affective purposes. Suddenly introducing a tight, enclosed audio effect or a cave-like echo while the player is in a wide-open environment, for example, can create tension. Setting audio and video against each other like this can be a useful technique.
Any related ideas?