Why won't iTunes add music from my computer to the library?

Hello, I am having a problem with iTunes adding music to my library from my computer. I am running the latest version of iTunes on my Windows 7 desktop and everything is working fine except for the add file/folder to library function. I have a folder with midi music files on my desktop that I want to sync to an old iPhone in order to play on my player piano. Whenever I try to add the folder or individual songs from the folder to my library the action of importing them just abruptly quits/fails to finish before anything happens. I used to be able to add songs from my computer without any problems but now it appears that iTunes isn't allowing that to happen. Is this because I cannot add midi music files to my library, or because there is something wrong with my version of iTunes that's not allowing it to add the music? Or this there a compatibility issue that I am unaware of? Does anyone know how I can add the music to my old iPhone (1st gen iPhone 5) in a different way if this is not possible or can point me in the direction of someone who would? Would I have to jailbreak it to make it happen? (I don't know anything about that I've only heard of people doing it before). Thank you so much for your time!