LOTRO features an 'ABC' music system which uses .abc music notation files to automatically (or manually) play music ingame!

Here are some resources to get started:

LOTRO Wiki: Music - details instruments, skills, and other ingame info related to Music.

Songbook plugin - an essential tool if you want to play music regularly. Organizes songs and commands into a UI box.

Archive of .ABC files - a mega-download of .abc files from defunct music sites.

Maestro MIDI to ABC converster - tool for converting MIDI files into ABC notation.

Meriband's Music Maestro Manual - a guide written for the Lorebook in 2012. Basic commands still useful.