Optical Isolators can be used for analog or audio signals.
You just have to understand the charactoristics of the device you are going to use.
A photodiode/ phototransistor pair functions much like a four terminal transistor - like having a seperate curcuit for the base current - not common with the emitter. All you have to do is bias the photodiode so that the phototransistor is operating in the center of its linear region.
Doug Hale
