Sipster no sound on calls

[fdl33333]

I have:

• installed sipster on raspberry Pi 3, "2019-04-08-raspbian-stretch.img"
• a working usb headset, tested with alsa, alsamixer, etc..
• installed latest pjsip, tested with test utilities, etc.. calls go through, audio works (bad mike audio ..)
  VErsions of node and npm:
  root@raspberrypi:# npm -v
  6.7.0
  root@raspberrypi:# node -v
  v11.15.0

I have written a test script (included here)... calls extension on working freeswitch server.
sipster registers, receives events, receives and makes calls, when tested with the "sound.wav" (now commented in code), other extension hears audio,
BUT no audio on incoming and outgoing calls.

INcluded code and log
Thank you for ANY help!
Francesco

sipster.log

t1.js.txt