זוהר בראשית — דף רמט ע”ב

Make your voice resound, daughter of Gallim, is said to the Community of Israel because she praises the Holy One blessed be He aloud. Whoever wishes to praise with a loud voice must have a pleasant voice that pleases others. The Levites who come from the left side must have a pleasant voice as befits them, for the spirit of the Levites' song is the spirit of the left. One whose voice is not pleasant should not stand to praise aloud, but should pray silently in the heart.