The in-game music is played on a 6-string guitar by Toady One himself. Partial tabs to the game song have been provided by Toady One on the Dwarf Fortress forum. This inspired LoopyDude to provide a complete fan made tab. A piano arrangement by Occoris/Sirocco is also available which layers additional parts together with Toady One's original.
Music can be turned off in the init.txt file, and the volume can be adjusted in the menu.
The Dwarf Fortress File Depot contains several alternative sound packs. Existing music is in .ogg format; replacing these files with your own music in .ogg format would allow your choice of music.
Gameplay sounds and more music are available by using SoundSense, which parses the gamelog.