Hi Calin
The output pin may be defined in the Illustron.b or Illustron.cpp. Maybe check in those to see if it is (and what value), as well as defining in your code.
Also, before that, there should be a sample program in the Arduino IDE or on this website for the tone() function. If you can find one, and configure it to use your audio output pin, it may help to test the circuit between the Arduino and amplifier.
All the best
Ray