I added a custom ambient music (ogg file) to a custom dimesnion of my mod.
There was a high chance that only my music played in this dimension. In this snapshot of MCreator 2020.2 my music either seems to play really rarely (normal minecraft music plays more often) or my music does not play at all in this snapshot. I tried to reselect the sound file multiple times, but nothing happens. I did not change the file since I first uploaded it into my mod (it worked before). (Category of the sound file in MCreator is "ambient".)
Maybe it is possible to give custom music a higher chance or to deactive vanilla minecraft music in dimensions once a custom file has been added to the dimension. (If code is needed, it can bee attached afterwards.)