CourtasanSatine said:
Lorne was and still is a riot, the fact he somehow talked Angel into singing is priceless.
But was his singing real?
From the link I posted above:
When "Buffy the Vampire Slayer" (1997) creator Joss Whedon saw Hallett singing in a Universal City blues revue, Whedon conceived the character of The Host, an anagogic demon who reads people when they open up through singing karaoke. Hallett was invited to try out and got the part, his first job ever as an actor.
A bit down from that:
According to an interview in the Cleveland Plain Dealer (April 24, 2003), "Buffy the Vampire Slayer" executive producer Joss Whedon had been friends with Andy Hallett and they liked to visit a "dirty, old scandalous" karaoke bar in Hollywood together; then Joss asked him to audition for the role of Lorne. Hallett joined the cast during the second season, but because he was unsure about how his flashy-dressing, green-faced character would be received, he kept his day job for an entire season.
I'd say, he was the one singing.
Edit:
and a bit down from that:
Hallett told the Cleveland Plain Dealer (April 24, 2003) that he loved singing while growing up in Osterville, Mass., but negative peer pressure made him quit. Then, at a Patti LaBelle concert, she invited him onstage to sing "Lady Marmalade" with her. When he received applause and requests for photographs, he realized that "It was really a life-changing experience."