Iroh frequently played the tsungi horn.
tsungi horn is a musical instrument used in the traditional music of the four nations. The curved and highly polished horn is cast from metal, and is believed to have originated in the Fire Nation.
Iroh was a talented player of the tsungi horn. He played it once on board Zuko's ship, and a second time in the  Jasmine Dragon.
 Aang had some experience in playing the instrument, though he admitted to not being very good at it.
 Zuko was very gifted at playing the tsungi horn, however, he adamantly refused to play on his ship's music night. 
 A member of The Flamey-Os played the instrument during the secret dance party in the beach cave.
The real life counterparts in terms of appearance to the tsungi horn are the sousaphone and the French horn.
The Track Team created the instrument's unique sound by superimposing characteristics of the trombone to the duduk, a double reed instrument from Armenia.
The musical instrument is sometimes heard in the score during the series, usually when Team Avatar is in sudden danger, such as when they stole the waterbending scroll from the pirate ship boutique, any time the  Blue Spirit appears, and most often during Zuko's themes.
The tsungi horn appears on the show more often than any other instrument.
↑ From older
Avatar: The Last Airbender official site, originally on Nick.com ( link). No longer updated, encyclopedia now broken though archived here.
DiMartino, Michael Dante, Konietzko, Bryan (writers) & Filoni, Dave (director). (June 17, 2005). " The Blue Spirit". . Avatar: The Last Airbender Season 1. Episode 13. Nickelodeon.
DiMartino, Michael Dante, Konietzko, Bryan (writers) & Dos Santos, Joaquim (director). (July 19, 2008). " Sozin's Comet, Part 4: Avatar Aang". . Avatar: The Last Airbender Season 3. Episode 21. Nickelodeon. ↑
O'Bryan, John (writer) & Dos Santos, Joaquim (director). (September 28, 2007). " The Headband". . Avatar: The Last Airbender Season 3. Episode 2. Nickelodeon.
DiMartino, Michael Dante (writer) & Volpe, Giancarlo (director). (November 18, 2005). " The Waterbending Master". . Avatar: The Last Airbender Season 1. Episode 18. Nickelodeon.
on Nicktoons Network. Avatar Extras for "The Waterbending Master"
↑ Acastus (2006-08-12 [date confirmed by the site's
news archive). Music Interview with the Track Team (part 3 of 3). AvatarSpirit.net. Retrieved on April 13, 2013. ↑
O'Bryan, John (writer) & Lioi, Anthony (director). (April 29, 2005). " The Waterbending Scroll". . Avatar: The Last Airbender Season 1. Episode 9. Nickelodeon.