Hi there,

after some trial and error, i finally got background music to work on custom maps.

Just save custom maps like the names of the maps shipped.
E.g. instead of saving "MyScenario.scenario" save it like "mp_serpent_chasm_2v2.scenario"
The names of the shipped MP maps can be seen via the archive viewer and the scenariosMP.sga file.
Ingame, your custom maps are still shown as you name them in worldbuilder under "scenario properties"
After exporting the map in worldbuilder, you can rename your custom map file and directory as desired.

I guess, somehow the game have some scripts for the standard mp maps which can not be succesfully loaded on custom maps but if they are named like the originals.