Rests would be history (pun intended)

The sentence below comes from a book, “Music and Cancer.”

http://books.google.co.kr/books?id=rnmCvU0F3pIC&pg=PT147&lpg=PT147&dq=%22rests+would+be+history%22&source=bl&ots=ke-Xf3W0hV&sig=LUxRlEyH-ouTmkGsyf4GwzLLzLk&hl=en#v=onepage&q=%22rests%20would%20be%20history%22&f=false

In this sentence, I can’t understand what the underlined part [“rests would be history” (pun intended),] exactly means.

When the writer says that "the rests would be history (pun intended),"

1. You (the reader) would know all the rest, that is, the song cannot be a song.

2. Because the writer said "pun intended," it would make a reader still think about the musical term "rest."

Am I right?

Because rests are silent, people often misinterpret these empty spaces as unimportant. But, imagine what would happen if a song was made up of only notes, and no rests. Aside from the fact that the “rests would be history” (pun intended), there would be a wall of sound with no reference point or discernible backbone to the music.

For you to know, I somewhat understand what “pun” means. For example, I fully understand those puns in the following thread.

http://forum.wordreference.com/showthread.php?t=1014352