[Quick feedback]
You've done a good job with this pluggin.

I tested from the demo only.
Comp. specs are:
Intel P4 2.4Gh
Creative Labs Sound Blaster Live
EAX2
DirectX 9c

The demo ran well and the sound transitions worked as coded. Two things you may want to work into the code though: First would be a trigger for threshold transitions. As is, the code triggers the outside environment sounds to stop as soon as the character enters a building or the pipe. A "threshold tigger" based on the distance that the character has moved beyond the entry point before the outside sounds are stopped would make the transition more natural (imo). This would also work in the opposite direction, when the character approaches the threshold trigger from inside the building. The second mod (related to the first) would be a fader affect that would reduce specific sounds. As an example, when the character enters the pipe and a threshold trigger is set, reduce the outside environment noise by a percentage while the footstep sounds continue at the normal volume. This would be applicable to any object that has a specific sound that would vary based on character distance from the entity. Another example, if a generator is running and the player character is close, the noise would be louder than if the character were further away.