It actually does say sos, the last dot just overlaps with the start of the lyrics so it's hard to hear. But you are right, the Morse code they put in the video is soi, although it annoys me when people don't use proper spacing
Hello! The music is from the link below :) ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-jzi6RNVEOtA.html&ab_channel=%E3%81%93%E3%81%A3%E3%81%A1%E3%81%AE%E3%81%91%E3%82%93%E3%81%A8-%E8%8F%85%E7%94%9F%E5%81%A5%E4%BA%BA