The trio appeared at the Apple Store in New York's SoHo to perform selections from Broadway's Carole King Musical.
The event is the culmination of six weeks of fundraising for Broadway Cares/Equity Fights AIDS.
Larsen currently stars in the Broadway production of Beautiful — The Carole King Musical.
Broadway creatives take listeners behind the music on their cast albums in a new Ghostlight series.
Mueller's Tony-nominated costars Jarrod Spector and Anika Larsen have also extended their contracts.
Campbell will replace Jake Epstein in the role of Gerry Goffin.
The new musical will open in the West End at London's Aldwych Theatre in 2015.
This presentation of the 2003 off-Broadway cult favorite will benefit the Hetrick-Martin Institute for LGBTQ Youth.
Kelli O'Hara, Jeanine Tesori, and Anika Larsen hope it can stay that way.
Neil Patrick Harris, Sutton Foster, Kelli O'Hara, Ramin Karimloo, and more traveled to the 86th floor to drink in the breathtaking view.