There was an excellent episode of In the Heat of the Night in 1991 called "Sweet Sweet Blues", which was a murder-mystery about a murder ballad. The crime happened 40 years earlier, and they solved it by tracking down the composer of the song.
The answer to the question "whodunnit" was a lyric in the song that the composer had kept out of all performances since soon after the crime. It's awesome how they work the music and performance into the act - a really fun episode!