* His own voice: He does have singing abilities and has released albums and performed live. However, in movies, his voice is often enhanced and manipulated.
* Autotune: This technology is used in recording to smooth out vocal imperfections and create a more polished sound. It's common in pop music and is definitely used on Zac's movie songs.
* Soundalikes: In some cases, especially when a character is supposed to be a great singer, a different vocalist may be used entirely. This is done to achieve a specific sound or if Zac's natural voice doesn't quite fit the role.
So, it's likely that while Zac Efron is singing in his movies, it's not always his raw, unprocessed voice. The extent to which his singing is enhanced or replaced varies depending on the film and the character's singing abilities.
Ultimately, it's up to you to decide how much you believe his singing is "really" his. Some fans enjoy the polished sound, while others prefer more authentic vocals.