Using ofxGui slider values outside of ofApp

you can create the parameter as a static variable like:

//.h
class Wave{
public:
...
static ofParameter<float> speed;
}

//wave.cpp
static ofParameter<float> Wave::speed{"wave speed",0,10,100};

in ofApp:

//.h
ofxPanel gui;
vector<Wave> wave;

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