He didn't sing at all in the first movie. Drew Seely provided Troy's vocals. Here's Zac's quote from Rolling Stone about it...
“I didn’t even sing on the first album. It wasn’t my voice in the movie. Even though I wanted to do it. So what do Du do when the entire cast is supposed to accept an award at the Billboard Awards and your voice is only on the album in a select few lines. I felt extremely guilty.”
As for HSM 2, HSM 3, and Hairspray he used his own voice in those movies, so if Du want to hear HIS voice, listen to the soundtracks for those movies. :]
