The sound themselves I think a fantastic. My issue if any is that they are all different volumes. In order to use then you have to change the sound settings each time you fly a new plane. Some are super loud and some are not, etc.
The quality I think is great unless someone more technical disagrees.
It's great quality.
Hard to say without knowing exactly the process he used,....but,..your description sounds like every file used needs to be individually "Normalized" to 0. This will put all the sounds/files at the same level, being 0. This is more time consuming than difficult, unless a batch option is available. Only after that should any mixing of volumes take place. This will bring more realistic sound immersion. If Normalize was not done, and then is done, there would need to be some remixing when he uses a few sounds in the same file, like an engine & prop noise mix.
That said, sliding off subject, I think I read where they were rendered in the .bank files as .ogg files....If so, I suggest the end product in the rendered .bank file should be .aac (lossless). .aac quality is almost equal to .wav, yet smaller file size (less compression). .ogg (high compression) is just slightly better then mp3. The more it's edited and saved the more quality damage it takes. Just a suggestion, nothing more.
Just happy someone gets to dig into them and make a few fixes.