Stereo has left and right audio information that are not necessarily both the same. 3D effects are accomplished by adjusting the volume of the left and right data independently of each other. Unless left and right data of a sound is the same, the result of the 3D effects are unpredictable. When I wrote the 3D calculation code for the JavaSound library plugin, I left that unpredictability up to the user to decide whether or not it was acceptable. The developers of OpenAL, though, simply prevented stereo sounds from being affected by positional information at all, limiting 3D effects to mono sounds only.