Question about Zac Efron???!

Question about Zac Efron???

I read in an article that Drew Seely sings all the parts for Zac in High School Musical. My sister also said that she heard this in an exclusive interview on TV. I also know that High School Musical became a live play that travels, and all the same actors/actresses from the movie played the same parts in the play, except for Zac. He wasn't in the play...instead Drew Seely, whom I mentioned before, played the part of Troy Bolton. My question is does Zac Efron really sing the part of Troy Bolton, or does Drew Seely sing it for him?

They blended the voices because Zac's was too low for some of the parts. I heard he's singing all of his parts for HSM2 and he sings everything in Hairspray.