The "Digital" opto-isolators like 4N35 and PC817 etc. will not work for audio circuits.
You need something that will be linear with a varying voltage applied.
FET's if biased properly can work.
Light Dependent Resistors work well also. You can use them as volume controls in a voltage divider circuit, with varying LED (or other) illumination. But you can use them as audio "Switches" too, if you switch the LED from OFF to Bright.
If you're into making your own for volume controls, then you could make switches too.
Google "LED-LDR" and you'll find how-to, like:
http://www.silonex.com/datasheets/specs/images/pdf/104057.pdf (A commercial unit)
If the coil Impedance (probably not the resistance) is 6K (did you say that?) then they should work well as switches.
Experiment!