If she sounded weird to you, it's because she's focusing more on getting the lyrics correct than the vocal aspect of the song. :)
I think she did pretty well considering that fact she sang all the parts and the transition from rapping to the singing was pretty quick.
Yes, Hyoyeon can sing. She needs longer parts in their songs! :x
The first clip is from their appearance on 11/23/07 and the second clip is from their apppearance on the show on 01/12/08.