Using ofxGui slider values outside of ofApp

I think you are using the :: wrong here. usually you use :: for static variable, here if you had a wave object, ie:

wave myWave

you'd access a variable called speed inside it like:

myWave.speed = .....

you wouldn't say wave::speed

there are sometimes times you would use ::, to access static variables like:

ofColor::red

but for generally accessing variables inside of instances of objects, you use . or -> if it's a pointer.