Version 1.13.0, 1.14.x or 2.x required to playback sound or music in the client.
Standard sound set v3Edit
There are two soundsets available, each containing a number of sound effects:
Freeciv 2.6 comes with an additional …/data/stdmusic/CullamBruce-Lockhart--Dawning_Fanfare.ogg as music_menu for its new …/data/stdmusic.musicspec feature. Up to Freeciv 2.5 CullamBruce-Lockhart--Dawning_Fanfare.ogg was used as music_start in the freesounds.modpack.
A number of music tracks have been created/released for Freeciv over the years. None is used by default in the client as of 2.6.0, but you can download and listen to them yourself: