As you may have figured, it ain't easy.
One of the problems with sound folders and files is that they are VERY subjective. What sounds good to you may not sound as good to someone else.
Check out the download sites that list download numbers for sound files.
Find a download for your type of aircraft that has had LOTS of downloads.
Try it out and see (or hear) how it fits your aircraft. If, for an example, the engine note just doesn't sound right you could try playing around with the sound levels in the sound.cfg file.
Be careful adding your own .wav files, the .wavs in FS usually need to fall into a range involving bit rates and compression.
As I mentioned, sound folders and files are subjective based on the people who use them.
This is why creating a sound folder tends to feel more like a "dark art". Its like the difference between writing a song and a number 1 smash hit. There really shouldn't be a difference, but there is. There really isn't a tutorial out there on how to write a config file from scratch, it involves a lot of guess work and testing.