Symbolism, Imagery, Allegory
Pasternak decides to include full versions of Russian folks songs at several points in this book. The songs always seem to have a timeless quality to them that helps Yuri Zhivago feel connected to Russia's history and make sense of the present by looking to the past.
Folk songs are especially good for this because they are like little time capsules that preserve the feelings of people from the past in a way that is still meaningful to people in the present. As Zhivago thinks to himself, "A Russian song is like water in a mill pond. It seems stopped up and unmoving. But in its depths it constantly flows through the sluice gates, and the calm of its surface is deceptive" (12.6.3).
In other words, Russian songs also give us a nice symbol for the spirit and substance that Zhivago sees underlying all of nature. He's felt this way ever since his mother passed away, and by listening to Russian songs, he feels intimately connected to everything around him. Even while the Communists are marching through the country and killing people, these songs survive.