I have learned that you can’t continuously play background music without having a persistence level. I think it’s unfortunate since this results in some problems, e.g. it basically disable the possibility to add user made maps since they must then be added to the “global” map in order to be possible to stream. This may work for a single map but becomes inflexible very fast (e.g. with two custom maps). Even though no modding is considered to be supported, it still doesn’t make sense to me to couple all your levels in one global level just to being able to play background music seamlessly.
One implementation would be to add sound cues to some global playlist that the audio module can handle in the engine or similar. That would solve this problem once and for all. An other option would be to allow maps to be streamed that doesn’t exists in the level as a sub-level. I would prefer the former over the latter though.
I made a question about this issue at answer hub, link.