Sure, Mosfets have parallel parasitic diodes (doesn't stop them oscillating though, given half a chance), but bipolars don't.

Steve,

in my archive I have an overwhelming number of schematics showing these free-wheeling diodes also in combination with bipolar transistors. They are even added in MOSFET circuits, although they already do contain them built-in.
But also bipolar power transistor modules which are used by 'Kenwood' and others contain them: By the help of digital multimeter you can measure this typical voltage drop.

It's a good argument, what you stated about this 'disappear in the air'. But that's only a small fraction. I know, calculation is very very difficult, because of this coupling to air. But keep in mind, that dampening of loudspeaker is not done totally by the air, but also by the amplifier! Why do you think that very low source impedance is needed for power amplifier, when dampening is already provided by the air? No, dampening of loudspeaker is provided electrically and mechanically. And the electrical dampening means nothing else, that the emf of the moving coil is acting with the very low source impedance of amplifier. And if for some reason the according power transistor is turned-off, e.g. because current limiting circuitry was activated for some reason (in order to achieve SOAR-protection), then emf of moving coil of loudspeaker must be shunted to power supply rail, via free-wheeling diodes.

Kai