But the most complicated thing is that I have the impression that these DLCs supersede the music of mods like Imperium Universalis.
No... it shouldn't.
The default EU4 music, including its DLC's should not conflict with any music added by mods unless the mod specifically aims to replace vanilla music tracks or has music with the same name. The only other possibility is the mod music being given a low chance to play by the modder; perhaps you can check the mod files if you suspect this.
Otherwise, you're probably looking to turn off the Guns, Drums and Steel/Sabton music packs, which, at the very least, you can do by unchecking specific tracks in the in-game music player.
Feel free to ping me in the Imperium Universalis Discord if you need further help; I have my head all around EU4's music as I work with it every day
