Here is the experimental library for BGT, called V3DSound. It relies on a custom DLL called v3d.dll used to manage 3d vectors that can be understood by the two supported sound systems. It provides BASS and FMOD full 3D simulation support, with position, velocity, etc. Not all features of FMOD or BASS are included and there are probably a ton of hidden bugs, but here is the link:
Link to 3d sound experiment
DLLs are included in the archive.
Also, here is a slightly older version of the library, considered to be the last stable version. It uses V3DSound by default. Link: Link to Heat Engine v0.9
EDIT: Warning, after reviewing this very old code, I'm pretty sure it isn't really usable or it doesn't use V3DSound even though it includes it! Long story short, this would need more work to integrate it cleanly. But basically, it proves I made attempts, and you can still use V3DSound separately to work with it. But Heat Engine still doesn't really support it.
The next version, v0.10, didn't have V3DSound shipped with it, probably because of stability/bugs issues. I don't really remember why. Also, on HE 0.10 I was working on a fully accessible level editor for HE games. In the 0.10 version this cannot be considered usable.
Also, I would considerer rewritting everything and possibily in another language... I think we should really all gather and build unique engine for our audio games.