The Zohar, Chapter “Shemot (Exodus),” Item 333: The ministers below are called “Levites” because they accompany and join above as one [Livui means accompanying], and because one who hears the singing becomes accompanying and his soul clings upwards, in the Creator.
A song is the revelation of the Light of Hochma in the Light of Hassadim, the greatest revelation there is. "The Song of Songs" is the peak of adhesion, connection, and unity between the creature and the Creator. Psalms are songs, which King David (Malchut) revealed in the darkness, they are revelation of the Light of Hochma in the Light of Hassadim.
Sometimes it seems to us that King David cries, not knowing what to do, and hides from enemies; yet in this darkness, he reveals Hassadim, the Creator’s mercy. Thus it is a song.
First we need to reveal the right line, faith or Hassadim, while suppressing the left line and holding onto the right one alone. When Hassadim are revealed completely, we rise into Bina and see that we can stay in mercy in all the desires, on this whole degree (in the desires of GE). Then we begin to work with the desires of reception (AHP) and rouse the left line. In other words, we begin to awaken the forces of Malchut and to accept the Light of Hochma in these receiving desires.
By elucidating our desires of reception and adjoining them to the right line, we thereby build the middle line. The state when the Light of Hochma is revealed in the Light of Hassadim, when the left line connects into the right line and gives raise to the middle line, it is called a song. This is called a "crusade" since we switch from state to state as a result of these relocations, thereby advancing by the three lines: Ibur, Yenika, and Mochin. We first prepare the desires, and then we fill them with Light.
Therefore, Psalms (the songs) of King David describe all the different states during the correction of Malchut in the three lines. They contain the immense Light that overspills the Kli, meaning that the correction is so high that the Kli becomes completely like the Light.
From the 1st part of the Daily Kabbalah Lesson 4/21/10, The Zohar