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, u can hear a very distinct voice from them. in the first chorus u hear sooyoung, in the seconde chorus u hear sooyoung and yuri, in the bridge u hear sooyoung, yuri, yoona and hyo, and in the third chorus u hear hyo again.
I NEED A TIME MACHINE~~ i'm a ballad lover, hehe.. this song is perfect.. i love it soooo much.. i heard that it was about -1C when yoona's cry scene.. yoona's acting.. JJANG!!