Les Troyens was an opera composed by Louis Hector Berlioz.
Captain Jean-Luc Picard listened to the aria Vallon Sonore from the opera Les Troyens in his ready room aboard the USS Enterprise-E during the Battle of Sector 001 in 2373. (Star Trek: First Contact)
The precise version listened to by Picard is contained in a set of live music by the London Symphony Orchestra, recorded on compact disc ASIN: B000063DQA conducted by Sir Colin Davis and released January 30, 2002 on Disc 4, Track 1: Les Troyens: Act V, No. 38 - "Vallon Sonore" at approximately 1:30 into the track