Niji no Oto (虹の音, Niji no Oto?, lit. "Sound of the Rainbow") is the ending theme song for Sword Art Online Extra Edition and also the name of Aoi Eir's 6th single. It is sung by Aoi Eir, with lyrics written by Eir herself, composed by Ryosuke Shigenaga and arranged by Hiroki Arai and Ryosuke Shigenaga.