Also, I've noticed that pretty much all SFX that originates from your own ship is much weaker in volume than with RC1 compared to external SFX; afterburner sounds, engine sounds, energy management sounds, targetting/aspect lock sounds... and of the external SFX, especially explosions seem really really out of bounds loud almost regardless of the distance.
This has been mentioned before, but I can't reproduce it. All of those sounds are just as loud as they used to be in my tests.
Another thing is that communications seems very low in volume as well, but I can't tell if this is some kind of 3D effect based on the position and distance of the transmitting ship - although if it is, I really think it shouldn't exist as the speakers/helmet headset move along with the player's ship, and so the volume of messages should be constant.
Messages are 2D sounds, so there should be no sort of 3D effect going on there.
I probably should test how it sounds with registry settings prioritizing General Software, but I'd like to know why a registry setting is used to change this instead of an INI file? Or is this just an intermediate solution?
Registry, INI file, same thing. It's just where the game settings go. It will eventually be moved out of the registry on Windows to an ini file just like on the other platforms though.
Also, should there even be any change at all between General Hardware and General Software with the new audio code?
So long as the device can support the number of sources that the game wants then it will use it. That is the only difference between how the game deals with different sound devices. How your system handles it is a completely different thing though. It really just depends on your drivers as to how all of that works.
Another thing, is there an upper limit to the sample rate? I realize that practically none of the files themselves used in FS2_Open have higher sample rate than 44.1 kHz, but setting that higher should still improve the volumetric sound calculations and other processing accuracy, and my sound card supports 192 kHz output to all 7.1 speakers...
The game has nothing to do with the sample rate other than to ask the sound device to initialize at a particular value. What works, or doesn't, depends entirely on what your hardware and drivers allow. However, the higher the sample rate the more CPU is required to do sound, so it is possible to overpower your system if you try and push it too much, particularly if you also enable EFX. If you raise the sample rate and then notice a huge hit to FPS, you'll know that you have to use a lower some.