Hallett had called 911 Sunday evening complaining of shortness of breath, said Ed Winter, a coroner's spokesman. The actor was rushed to the hospital, where he was pronounced dead at 8:35 p.m.
An autopsy is pending. His death was first reported by E!Online.
Hallett was born in 1975 in the village of Osterville, Mass., part of the town of Barnstable, on Cape Cod. He attended Assumption College in Worcester, Mass., and studied business.
Hallett told the Cleveland Plain Dealer in a 2003 interview that he loved singing while growing up on Cape Cod, but his friends discouraged his efforts. Then, Patti LaBelle invited him onstage to sing "Lady Marmalade" with her at a concert. When he received applause and requests for photographs, he realized, "It was really a life-changing experience."
Hallett moved to Los Angeles, where he worked as a runner for an agency, property manager and then personal assistant.
He was singing in a Universal City blues revue when he met and became friends with Joss Whedon, creator of the television shows, including "Buffy the Vampire Slayer" and its spinoff, "Angel."
It was Whedon who asked Hallett to audition for the role of Krevlornswath of the Deathwok Clan, known as the Host and Lorne. The character was the consummate greeter at a karaoke bar named Caritas, where Lorne was able to read people by how they sang.
Hallett played Lorne in 76 episodes and sang in six of them. In one episode, he sang "Lady Marmalade."
Information on survivors and services was not immediately available.