While they are singing the song "A girl worth fighting for", near the end of the song, Ling sings "Wish that I had" and it sounds nothing like his previous voice. In fact, it sounds just like Mulan's voice! I found this kind of strange and am surprised more haven't noticed it.
Did anyone ever stop to think that this might be meant to be funny? Guys speaking (or in this case singing) in a falsetto voice is sometimes put into a movie for humor.
It's the same voice. How could it possibly change?! He was singing that part of it differently! The expression on his face is very "dreamy" so obviously that is the way it was sung by the voice actor and the cartoonist added that expression to magnify that change. It's just like when Shang was singing "And you can BET..." He was hit in the stomach with the stick and the actor was told to sing the word "bet" differently. Get it?
Um...no! That just flat out doesn't happen! His voice gets a little less agressive and more clear - like the other post said, kinda "dreamy" - because he'a thinking about his dream girl, but it's obviously his voice and it sounds just like his voice, just less raspy!