Digital Optical Volume Control with LED and photoresistor (light to seperate circuits, optical pot)
Admirable attempt, and good debugging effort. Your Cannon opto-couplers are different animals than those described in the link you provided. They do use an infra-red diode to provide the light, but the photo-sensitive element is not a resistive one, but is a silicon photo-transistor. The current it provides only flows one-way - hence the clipped waveforms that yield highly distorted audio.
The photo-sensitive element in your link is a cadmium-sulphide or cadmium-selenide photoresistor. It is a resistor that allows current in both directions in a reasonably linear way. Search terms like photocell optocoupler might help.
Some currently available part #'s: NSL-33-007 from LUNA optoelectronics is a dual-channel photocell optocoupler.
NSL-32H-103 from LUNA optoelectronics is a single-channel version.