Using ofxGui slider values outside of ofApp

the easiest, and cleanest way to use this is to use ofParameter to represent the speed and then pass that to a panel which will create a slider automatically. take a look at guiFromParametersExample but it would be something like:

class Wave{
public:
void setup(){
    speed.set("wave speed",0,10,100); // value, min, max
}
...
ofParameter<float> speed;
}

in your ofApp:

//.h
ofxPanel gui;
Wave wave;

//setup
wave.setup();
gui.add(wave.speed)

if you are using 0.9.0 you can also initialize parameters in the .h file directly like:

class Wave{
public:
...
ofParameter<float> speed{"wave speed",0,10,100}
}