This paper describes an integration of a Head Related Transfer Function (HRTF)-based 3D sound convolution engine into the open-source VoIP conferencing software Mumble. Our system allows to virtually place audio contributions of conference participants to different positions around a listener, which helps to overcome the problem of identifying active speakers in an audio conference. Furthermore, using HRTFs to generate 3D sound in virtual 3D space, the listener is able to make use of the cocktail party effect in order to differentiate between several simultaneously active speakers. As a result intelligibility of communication is increased.