Quote:
Any pointers? (Pardon the pun)

Those C++ member functions are not compatible with normal C functions (because they get the "this" pointer passed to them). If you want to pass C++ member functions as C callbacks, they must be static member functions. If you pass the address of your class as the "client" pointer, you can then call member functions inside your static callback functions:

Code:
class MP3Play {
 static void process_callback(int nframes, void *clientptr)
 {
    MP3Play *me = (MP3Play*)clientptr;
     me->callback(nframes);
 }

 void callback(int nframes) ...
};

jack_set_process_callback( &MP3Play::process_callback );


Peter