when I open my music app and touch a song, it quickly jumps to another song and won't play.
The file is corrupt or belongs to another iTunes account.
Delete the song and download it again.
This could be because of a corrupted music file. I recommend resyncing your library, or finding the corrupted file and deleting it.
If it's corrupted in your library it will do the same thing as it does in your music app when you try to play it. That's how you can find it and delete it.
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