I have found the following code in
_calc_output_vol(local_pos.normalized(), 4.0, output);
Information about the second parameter is given:
The lower the second parameter (tightness) the more the sound will "enclose" the listener (more undirected / playing from speakers not facing the source) - this could be made distance dependent.
Added a new property for the second parameter. When the value is zero, the sound has no directivity. This should be a function of the distance but works well for now. Hope this will get implemented soon.