Baldwin, whose brother Alec will portray Dennis Dupree in the Broadway adaptation, said that audiences will be pleasantly surprised by Cruise's singing voice when the film is released next year.
"The word behind the scenes is 'intergalactic'," Baldwin told People yesterday.
"He covered a song by a particular band, and the feedback was that the band was completely blown away," he explained. "The whispers and the rumours are that what he's doing with Rock of Ages is out of this world. Pretty cool, huh?"
Cruise recently posted the first photo of himself in costume as rockstar Stacee Jaxx, complete with a glam-rock hairstyle and several fake tattoos.
The Adam Shankman-directed musical also stars Russell Brand, Catherine Zeta-Jones, Malin Akerman and Bryan Cranston.
Rock of Ages is due for release in cinemas on June 1, 2012.