Music will shuffle every assault wave, the track will never repeat consecutively.
The playlist contains all your tracks by default, you can choose which payday 2 tracks are allowed in the menu options.
(Options -> Mod Options -> Shuffle Play)
Here you can select which payday 2 tracks are allowed in the playlist, it is ordered alphabetically for ease of use.
Exists two more options: one to toggle the mod (at anytime) and the other to toggle BeardLib tracks load.
For people using BeardLib for custom tracks.
At the game launch, BeardLib songs will be loaded so the mod can switch songs without the game freezing.
You can disable this in the mod options (as referred above), but BeardLib tracks will be removed from your playlist.