I'm having trouble getting Volumetrics to respond to the World environment lighting. I'm using a standard node setup combining Volumetric Absorbtion and Volumetric Scatter and it appears to work as expected for standard Lamps and also for Mesh lights but not for the World lighting?
There is a checkbox on the World Ray Visibility to control visibility for Volume Scatter rays and that appears to have no effect (see the checkbox being flipped in the right-hand panel but no effect on Suzanne) :
To show that this is working for standard (point) lamps and for mesh lights but not for environment lighting :
The sequence in the above image is all on, lamp off, lamp on, mesh off, mesh on, environment off, environment on. You can see that the lamp affects the suzanne volume, the mesh affects the suzanne volume, but the environment does not affect the suzanne volume.
What am I missing here?.... Or is this a bug?
I've tried this on Blender 2.76b and 2.78a with the same results.