When you have a strong feeling that you have heard a familiar song before and you associate it with a time or an emotion, but you aren't exactly sure when or where you have heard the song. This usually occurs most strongly at the beginning of the song, but can last the entire duration as well.
When the otherwise seemingly happy song came on, Paul felt nostalgic and a bit depressed, although he wasn't sure why. He was experiencing musical deja vu.
I hate it when I get musical deja vu because I can't remember where I heard the song before.