Location of iTunes Music

First, I know that I cannot view or play all my music in iTunes simply from the fact that many artists that show in Source 2 are not showing up in iTunes.Are you saying that S1 and S3 are added to iTunes and play OK, while S2 is only partly added?iTunes Music folder Location: c:\Users\Owner\Music\iTunes\iTunes Music.So, this is empty or what?My first question is why the three sources are showing different results?That's something only you can answer. You put the files there.The second question is how do I get my system back to recognizing, showing and playing all the music that Source 2 above represents?I'd try the consolidate command. That would take all known songs and put them in a folder of your choice. Then you'll manually have to sort out what's missing from the S2 folder and add those songs to iTunes. When all looks A-OK, you can delete S1-S3.
Alternatively, start with an empty iTunes folder and library and add everything from S1-S3. You'll lose playcounts, ratings etc but you'll have more control over what gets added.