The choruses are the part of opera most people can hum without knowing where they come from. The Nordic Opera Choir and the Elja chamber orchestra spend an hour on them, and on the arias that travelled the same way, under Ragnheiður Ingunn Jóhannsdóttir. The Anvil Chorus from Il Trovatore, Va, pensiero from Nabucco, the Humming Chorus from Madama Butterfly, the Habanera from Carmen, and less familiar music by Borodin, Kaija Saariaho and Helena Munktell. A reasonable place to start if you have never been to an opera.