If you need to electrically isolate an audio signal path, decent audio transformers aren't that expensive these days. A Triad Magnetics TY-250P is less than €6

If you need to electrically isolate an audio signal path, decent audio transformers aren't that expensive these days. A Triad Magnetics TY-250P is less than €6 at Mouser in single quantity, and is for sure a lot more linear than most optocouplers. https://eu.mouser.com/datasheet/2/410/TY_250P-1892894.pdf https://eu.mouser.com/datasheet/2/410/TY_250P-1892894.pdf