The Brute and Grunt music is played at priority 0 (The highest priority, meaning it will play over any and all other music).
So, add an interact callback that stops the music at priority 0.
That being said, the music really isn't that loud so you should look into some sound editing software to raise the volume of the voice.
Here is what the code should look like:
SetEntityPlayerInteractCallback("inGameNoteName", "StopTheme", true);
//Your other OnStart functions, callbacks, etc
//Then put this anywhere else, outside of all other functions
void StopTheme(string &in entity)