z axis volume problem with DirectSound listener

Tech Tip: Click here to run a free scan for Windows Errors and optimize PC performance

I'm playing a positional sound from the origin. As I vary the z-coordinate
of the listener, the volume of the sound peaks around z = 2.4 rather than at
z = 0. It's 100% consistent and easily reproducible. This is with the
February 2005 release of DirectX 9. Has anyone else seen this? Is this a
bug in DirectSound?
.