It's a mono sound and is coming from a specific direction.
For stereo output:
If it's exactly right of the listener - however close or far, it will be only in the right 'ear'.
If it's exactly left, only the left 'ear'.
If you want to hear the sound in both 'ears', it needs to be in
front of the listener (or behind).
Basically, put the z value into positive values. I've found that it should never really be exactly zero (unless you need exactly only one side); it's too unpredictable. A very low positive value works well but if you're moving around the stereoscape, it should be in a relative scale to the movement of the x value.
For example, try a sound's position of { 0.5f, 0.f, 0.866f } (this should be about 30 degrees to the right, starting from in front of you as 0 degrees)
Have a look at this that shows sound orbiting around the listener:
https://github.com/Hapaxia/OrbitingSound