Using ofxGui slider values outside of ofApp

the easiest way to do this is either pass the value through to the wave object from ofApp, or in the .cpp file of wave (you can't do this in the .h file) you can do:

#include "ofApp.h"    // note you can't do this in wave.h b/c it would be a recursive include...

then later in the cpp you can do:

((ofApp*) ofGetAppPtr())->waveSpeed

ofGetAppPtr() gets a pointer to the app that OF is running (ie, ofApp) and (ofApp*) casts it as an ofApp (it's a pointer to an ofBaseApp). I use ofGetAppPtr() to get access to the main level of the ofApp inside of other classes, like what you are trying to do here....