Wilkie1: Check the orientation of the opto-coupler. I think you have it installed backwards. The LED leads need to be facing up on the photo and the + lead must be placed on the pad marked +.
It’s true!! I welded it as it is done with it’s pins spread on one side and tight on the other.
It is now in the right direction, and… No sound effect too. Could it be broken?
Maybe I should try another audio tests with my audioprobe.