Why do dirty switch contacts cause harmonic distortion?

[living sounds]

Coils or contact surfaces failed? Sockets can get wonky too and need replacing.
Latching relays won't need much coil power

These relays are fully encapsulated in black plastic, so there is no way to know the how/why of the failure, unfortunately. There is hot switching going on all the time in a console, never a good thing for a relay.