I have investigated a little and yes, the actual sounds (engines, warning, outside view etc.) are in .PCK files. There is also a sound.xml file which defines how the sounds are used, a control file.
Now changing single sounds is very hard. Why? Because the PCK file has to be extracted, sound have to be replaced. They are not in .wav format, but other formats that are common in the gaming industry.
Even if you replace the sound files, you still have to compile them to a new .PCK file. And now comes the problem. Without the actual project file, that only the developer owns, it will be very hard to create a .PCK file and sound.xml because you need the project file that stores how all the sounds work together, how they are played etc.
Sooner or later, 3rd party devs will find out how to make .PCK files, but it is nowhere as easy as it was in FSX or P3D.
Even if a sound dev manages to create the .PCK file, there is still the legal issue. Does he even have the right to sell a modified soundpack to clients? I think only if he uses ONLY HIS sound files. But this won’t be the case because in most cases, only certain sounds will be replaced and the rest will be used from the original soundpack.
So, not an easy situation. I hope there will be many good soundsets, though.