Take a look in the settings.lua which is located in the locale subdirectory. (e.g. germany, singapore, ..) Scroll down, add
Code:
add_bgm_info(map_index, sound_file, 0.5);
and replace "map_index" and "sound_file" with your map index number and the name of your sound file. Move the corresponding sound file to the BGM directory of your client then. Restart the game server to take effect.
Greetings