When you have a callback, two arguments are always passed to it: source, and event. Source is the handle of the object that generated the callback, and event has details about the event that was triggered. No other arguments are passed by default: any other arguments must be passed explicitly. When you are using GUIDE, GUIDE generates the callback to not name a routine directly and instead to execute code that finds the "handles" structure and pass it as a third argument to the callback routine.
Thus, varargin{1} in your routine is the handle to the uicontrol (same as S.play). handles are datatype "double".
It looks to me as if Play_Music_Call is possibly a nested function. If it is not nested at the moment, you could convert it to nested by adding an "end" after the Play_Music_Call definition
function Beam_Deflection_GUI_3_Music
[....]
function Play_Music_Call
[...]
end
end
Once it is nested, then remove the assignment to S in Play_Music_Call, and then that routine will have access to the values assigned to S in Beam_Deflection_GUI_3_Music
