Another way of thinking about that is that the material is there, but the problem is access or retrieval. The neighbor's playing leads you to the "lost" song.

Access and retrieval of tunes and of words of a song are different problems, in my experience. They are stored at different places in your brain, or maybe it's "stored with a different coding". I find that usually a tune will come back essentially whole, but the words are harder, if it's a long connected series. But if there's a strong story line it's a lot easier.

Dave Oesterreich