Another way I was thinking at is to let FlightGear handle the sound generation but let aircraft developers influence the sound properties like (change in) volume.
That looks nice...
I guess ideally we'd offer three possibilities:
* sounds generated by an FG environment ambience system, passed through default settings
* sounds generated by an FG environment ambience system, passed through customized settings (say, volume attenuation,...)
* sounds modeled completely aircraft-side with FG environment ambience off
(which basically just requires a flag that switches the FG-side things off).
The reason I'm advocating this is that I really see that there's lots of detail which potentially matters. For instance, rain on a metal fuselage sounds different from rain on a canvas wing of a WW-I plane. Thunder may echo differently in a metal fuselage. Birds are usually not disturbed by sailplanes, so you can hear them as you soar in the same thermal, but they're unlikely to stay near a heli. And so on.
I think it would be nice to add environment sounds to a lot of planes for cheap, which we get if we generate it FG-side, but it should be possible to optionally get all the details.