if the music is that loud that is a huge distraction.
if they cannot hear what's going on around them then the music has to go.
it won't be a problem until like Dexter said, when someone gets hurt because of it THEN it will be a lawsuit problem.
I guarantee if you ask any safety officer in any plant they will most certainly tell you this.
Driving a forklift is sort of like driving a car, same rules apply but 1000% more stringent. Cars do give protection, forklifts give very little if you compare the two.
Hearing is most certainly a major factor when operating in a warehouse environment especially if your driving an electric lift.