I am not an audio expert, and those circuits, 324 op-amp and all, hurt my eyes. That said, I didn't look close, and they might be fine for what you might call

I am not an audio expert, and those circuits, 324 op-amp and all, hurt my eyes. That said, I didn't look close, and they might be fine for what you might call "utility audio". The IL300 at least has defined linearity specs. If you do audio design you'll be better qualified than me to comment on them. But I can say: remember, too, if you're using these that they're basically current devices, the transfer ratio across the gap is going to be all over the map in production, feedback is another design challenge, and the saturation voltage will be high, all over the map, and basically the point where more light will not turn on the transistor any more. If it's a hobby project, have fun, that's the point.