I think the researcher is arguing not that mushrooms are responsible for the sound of existing Stradivarii, but that they can be used to create new violins with

I think the researcher is arguing not that mushrooms are responsible for the sound of existing Stradivarii, but that they can be used to create new violins with similar characteristics: The mushroom nibbles lightly on the wood, reducing its density and intensifying the sound, he said. [...] Many researchers believe the unmatched sound of Stradivarius violins comes partly from the instruments' age.