What qualifies as song copyright deposit?

Dear Rich: When you copyright a song (not a recording) what materials do you really need? Does an audio recording of the song qualify or do you need a lyric sheet/sheet music in written form or any combination of the aforementioned? We assume you're asking about the materials you must deposit when you are registering a musical work (or "song") copyright. The answer can be found in Circular 50, Copyright Registration for Musical Compositions. If your song was never reduced to sheet music (probably the case for 90% of the songs written these days), then a CD is fine. (Alas, the Dear Rich Staff is so old that we remember when the rules were different -- yes, a time before CDs and computers -- when we had to pay a music student at the Indiana University School of Music to reduce our songs to sheet music.)