How do I add custom DEFAULT music?

[MW2917]

Here's the deal: I wanted to define a new internal music for a game mode.
I tried to have a sound defined in side a game module, but that didn't work.

IMG_0236_compressed.mp4

I also tried to just add the music type on the _music file, but that obviously won't work as it didn't know what even is it (Why did I even tried?) So I wonder if there IS a way to do.