You can add the files to your music player, then in the music player add each song from one of the 7 folders in a separate playlist, 7 times. It will take FOREVER but that's about the only way I can think of. also, if you want to, you can create a playlist icon to put on your home screen, so you can add all seven playlists as a separate icon on your home screen.
If you do that, you can save space by long pressing an empty area on your home screen, and adding a blank folder. Tap once on the folder icon (it will be white and say "Folder") to open it, and just drag and drop each playlist icon into the folder. So then you will have 1 folder containing all seven playlists.