The Doctor performing for the Qomar
Opera is a dramatic form involving a performance set entirely to music with sung dialogue. They are often performed in opera houses. Among the planets that have developed opera are Earth and Qo'noS.
The Doctor, USS Voyager's EMH, was particularly fond of this craft and added information and subroutines to his program to make him an opera singer. Due to this massive amount of info, his holographic matrix was in danger of destabilizing, but was ultimately repaired by merging it with the matrix of the EMH diagnostic program. (VOY: "The Swarm") Captain Kathryn Janeway, however, may not have been particularly fond of opera, as she told The Doctor when inviting him for coffee in Buenos Aires on the holodeck in 2378 that there were two rules: "leave your rank at the door" and "no opera." (VOY: "Renaissance Man")
While patrolling the Romulan Neutral Zone in the USS Enterprise-E, during the Battle of Sector 001, captain Jean-Luc Picard listened to an opera by Berlioz. Commander Riker entered his office and mistook it for Bizet. (Star Trek: First Contact)
In 2376, The Doctor introduced music, including opera, to the Qomar who were initially fascinated with him and treated him as a celebrity, but, as they developed their own style of opera, they replaced him with another hologram whose register was much wider. (VOY: "Virtuoso")
Kelemane's planet also developed opera. (VOY: "Blink of an Eye")
A scene in the script of "The Inner Light", but not in the final episode mentions that Picard once spent nine hours at the opera, watching the Ring cycle with admiral Gustafson.