Audio 5.1 no rear speakers

Hello.

I'm having problems playing sound as a 3d-sound in unity. I animated a sphere flying through the camera from front to back and assigned an audio clip.

When I play the scene I can hear the audio clip in the front speakers only but not in rear speakers.

Anyone has an idea what I might miss ??

Speaker mode is set to surround 5.1 and also driverCaps tells me 5.1 is available. The audio clip is imported with 3D checked and is a mono wav file. Tested on osx yosemite.

However when I use a native 5.1 audio clip and import as none 3d, this sound is played through all speakers.

thanks