Without seeing the class mySynth
code it is hard to say but this error is commonly encountered when you have a class containing a pure virtual function. Either that or you have derived from a base class with a pure virtual function and have failed to override it with a derived class implementation.
If you do not know what that means, look in your class (and sub classes) for functions declared like this
virtual int my_function() = 0;
This kind of function is a pure virtual function and a class that has one is considered an abstract class and cannot be instantiated. In order to do so you would need to provide an implementation.