I doubt the sound has anything to do with mushrooms. For one, sound quality is a subjective thing, and to speculate that these particular mushrooms improve sound quality in exactly the right way to produce a Strad sound is really stretching it.

For another, I heard that the wood used to produce a strad was from logs that had been kept chained under salt water for a large number of years. This produced a "partial fossilization" or something. I'd think that it would have a fairly strong anti-fungal effect, too.

Any way, I've heard so many things that could be the reason for the Stradivarius sound... old growth wood, special varnish, etc... It seems that once every year or so someone discovers the what causes the sound.

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.

I've heard that it was a mini ice age, induced by a big volcano eruption spewing ash into the atmosphere, that's responsible for the Stradivarius sound.

It cooled the climate and slowed down the growth rates of the trees, slower growth - denser wood - better sound.

But as you say, there are so many theories about the Stradivarius sound, coming up with new ones must be a sport.

Yep! Everybody has an educated guess, and so far most of them haven't helped to replicate the sound.

Hopefully one day somebody'll figure it out. A lot of what makes the violins sound good has to do with the musicians playing it, but the sound is still incredibly rich and wonderful.

Sorry but this is a fungus, not a mushroom. A mushroom is only the reproductive body of certain fungi.