I was playing a tune yesterday morning, and when it finished I hit the left arrow to play it again - this time it sounded like a harsh scratching effect - similar to a tv channel that doesn't have reception. I've tried to replay the song a few times since then and it still sounds like hell - same harsh sound - it seems as if the mp3 file is now damaged.

Have you tried to play that song again since the 'backward' incident? If so, does it sound ok?