Okay, past tense is not my strong point, but I think it should be:
"I labbri che hai baciato sussurrano per piu'."
Che is used instead of quello in this case, because quello is used as a pronoun, as in "That is her", whereas che is used in demonstrative clauses like the one that you wrote. The rest of it, I can't explain, because I'm not sure whether mine is even correct. You are really challenging my limited italian knowledge. Did I mention that in my italian class we haven't learned anything but present tense so far?
Anyway...
Sento i tuoi sussurri, ma non capisco le tue parole!
(I hear your whispers, but I don't understand your words!)