Trying to move music between Apple devices

Years ago I copied a CD collection onto iTunes via a windows PC. They no longer show up in iTunes on the PC and I can't access them with the Music app on our 2 iPhones and 1 of our iPads. However, they are still in the Music library of my husband's iPad and can be played from there. I would like to move them back into iTunes so we can access them with our various devices, but would really be happy if I could move them onto the PC. There's about 36 albums so I really don't want to re-copy them. I can't find where the files are located or anyway to share them from the iPad. Also, when I started this mission my husband's iPad would show up in iTunes on the PC but the albums didn't show up there. Now the iPad doesn't even show up either.