K4003 Diagram doesn’t stem with the TDA2616 IC and the pc board. There is a ‘Mute’ control in pin 2 of the IC that is not shown in the diagram nor explained in the text.
The schematic was not updated when the IC was changed last year, you can always download the datasheet from the TDA2616 when you need more info.
From where? How does it tell how the ‘Mute’ K4003 is to be used?
It’s very simple, if you close the contact, the sound is muted.