The contents of this article are non-canon to the Avatar franchise (source).
The Cavern of Songs lies at the heart of Willowglade in the north west of the Swotulu region. It contains a sacred willow tree which contains one of the four harmonics needed to locate the Well of Souls.
The sacred willow tree in the center
With the three unobtanium shards in hand, Able Ryder and Lungoray entered the cavern to try and draw out a harmonic. An RDA sniper killed Lungoray before this could be done, but Tan Jala avenged him and Able Ryder was able to position the three shards around the cavern and to draw the harmonic.