Anyone care to speculate?

Seems like it is explained.

http://en.wikipedia.org/wiki/Sonoluminescence

Seems like it is explained.

http://en.wikipedia.org/wiki/Sonoluminescence

Not quite.

The mechanism of the phenomenon of sonoluminescence remains unsettled. Theories include: hotspot, bremsstrahlung radiation, collision-induced radiation and corona discharges, nonclassical light, proton tunneling, electrodynamic jets and fractoluminescent jets (now largely discredited due to contrary experimental evidence).

So while it seems the light is due to the temperature, the way I'm reading it, it's still unclear what causes that.

 

I vote aliens. Maybe the same ones that made the cross on the Sun.

The strong tendency is to consider heat from the compressed gas as a sufficient explanation, especially if the gas is mainly monoatomic.

 

Though, detailed observation is difficult because a part of the light spectrum is stopped by water, so better observations may promote different explanations when available.