The commentaries discuss to which song Moshe was referring. The question is whether Chazal consider the entire Torah to be a song, or whether the reference is to a specific part. Why is the Torah called a song? Simply put, the Torah represents harmony, a unity of perfection in which every phrase, every verse, every mitzvah is perfect. The Torah is not vulnerable to reform or change. Because it is a Divine composition, it is perfect in every sense. Just as a musical score loses its flavor if one makes a single variation in the notes or rhythm, so, too, the Torah collapses, if one removes or changes a single mitzvah. It is no longer the Torah.
Horav Moshe Swift, zl, suggests another idea why the Torah is called a song. The Torah brings music to life, it gives it meaning and harmony, it transforms the most sorrowful event into one of hope and even joy. With Torah one can cope, one can hope, one can begin to understand. Without Torah, who really are we?
Moshe instructs Klal Yisrael to write the song, to study it and teach it to their children, placing it in their mouths. The melody of Torah should always be on their lips; they should sing the Torah, and they should live by it, so that it will bring joy to their lives.
