Interesting idea, but video monitors aren't designed to handle that sort of
thing. Your typical vertical deflection circuit is designed to work at 60
Hz, and the horizontal deflection circuit wants to run at 15.7 kHz (or even
twice that frequency, if it's a VGA monitor). Going too far beyond those
boundaries creates all kinds of hellacious noise, and/or fried components.

It would probably be easier to use a cheap X/Y oscilloscope instead. They
are designed to handle just about anything, and you can plug your stereo
outputs directly into the 'scope inputs with no trouble at all. Any
adjustments for optimum picture can be done right there from the front panel
of the 'scope.