Teensy 3.1 IntervalTimer code within class problem

[Theremingenieur]

Although it might rather be a CPP problem (or better a "me understanding CPP" problem), I allow to ask here since it's a teensy 3.1 project:

My project is simple: I created a class which generates a 2kB wave table (depending on several parameters) and everything worked fine until I wanted to output the wave table with the help of an IntervalTimer (private object within the class).

Here is the header declaration of the class from the .h file:

class THF_son
{
  public:
    THF_son(thfInstruments_t monInstrument, thfPresets_t monPreset = thfPresetDefaut);  //constructor
    void setIncrement(unsigned int monIncrement = 1000);  //Method to change the output frequency by setting an increment for the table pointer
    void demarre(); //Should start outputting the sound (gives a compiler error, see below)
    void muet();      //Should stop outputting the sound
  private:
    void _genTableau();  //private Method to generate the wave table, called by the constructor (work!)
    void _sortie();          //private function to read a value from the wave table and to output it to the DAC, to be called as ISR from the IntervalTimer
    IntervalTimer _ticOut; //THE Intervaltimer
    volatile int _tableau[thfTailleTableau]; //the wave table itself, volatile to be in reach from the ISR
    volatile unsigned long _pointeur = 0;  //the wave table pointer, volatile to be in reach from the ISR
    volatile unsigned int _increment; //the wave table pointer increment (set by public method setIncrement), volatile to be in reach from the ISR
    thfOndes_t _onde;    //private internal parameter for the wave table generation
    thfFormes_t _forme; //private internal parameter for the wave table generation
    int _relamp = 1;       //private internal parameter for the wave table generation
    double _par1 = 0, _par2 = 1, _par3 = 1; //private internal parameter for the wave table generation
};

Now the excerpt from the .cpp file:

void THF_son::setIncrement(unsigned int monIncrement) {
  _increment = monIncrement; //sets a new increment
}

void THF_son::_sortie(void) {
  int y = _tableau[_pointeur]; //reads the wave table value at position _pointer
  unsigned int z = y + (1 << (thfResolution - 1));    //adds an offset to pass from signed to unsigned
  //Todo send z to the DAC
  _pointeur += _increment; //moves the pointer forward by the increment
}

void THF_son::demarre() {
  _ticOut.begin(*_sortie(), thfCadence); //<=== gives a compile error:
[COLOR="#FF0000"][B]THF_son.cpp: In member function 'void THF_son::demarre()':
THF_son.cpp:34: error: void value not ignored as it ought to be
[/B][/COLOR]}

void THF_son::muet() {
  _ticOut.end(); //should stop sound output
}

When I take away the brackets from *_sortie as follows, I get a different compile error:

void THF_son::demarre() {
  _ticOut.begin(*_sortie, thfCadence); //<=== gives a compile error:
[COLOR="#FF0000"][B]THF_son.cpp: In member function 'void THF_son::demarre()':
THF_son.cpp:34: error: invalid use of member function (did you forget the '()' ?)[/B][/COLOR]
}

Please help! I feel that the solution is simple, but I'm not familiar with pointers...
Thank you in advance!