Oh, dear... so baaad.
I think the reason why some of the girls' voices sounded off or strained is because either (a) they were projecting their voice more to make up for lack of mic or (b) they couldn't hear the back track properly, or both. Although, for some of the girls, you have to wonder... I love Sunny so so sooo much (second favorite girl after Hyo) but her voice was off. I think you were hearing Seohyun's backtrack, kimmeh, because whereas with Taeyeon, you could hear her singing over the backtrack (and it sounded really good!), I couldn't hear any overlapping voice(s) with Seohyun.