actually, they do sing, it look me time to figure it out, but the 4 main dancers sing in the chorus and in the bridge, you can hear a very distinct voice from them. in the first chorus you hear sooyoung, in the segundo chorus you hear sooyoung and yuri, in the bridge you hear sooyoung, yuri, yoona and hyo, and in the third chorus you hear hyo again.
I NEED A TIME MACHINE~~ i'm a ballad lover, hehe.. this song is perfect.. i amor it soooo much.. i heard that it was about -1C when yoona's cry scene.. yoona's acting.. JJANG!!