Although Kevin Alejandro has not had the opportunity to sing on Lucifer before the musical episode, that is indeed his voice fans are hearing during Dan's musical numbers.

Who is Lucifer's singing voice?

Tom Ellis (Lucifer) does his own singing throughout the show, although does not play the piano on screen. In early interviews for the show, he said he didn't "really know" how to.

Can maze really sing?

This includes Lesley-Ann Brandt, who was indeed singing during the musical episode. Fans likely won't be surprised to learn that Brandt is actually singing in the Lucifer musical episode as she's performed before on the series and has been vocal about her passion for singing.