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.

This is a common sense issue. I think the CFR 1910.178 is where you should look

Some warehouses or factory floors are so noisy you need to wear earplugs Some are not. rock and hard spot issue. Make everybody mad when they are allowed to listen to their tunes ;or when somebody gets hurts and sues because they could not hear the motion sensors. I think crown has a really loud motion alarms in their parts system, plus when they install them you have a little more piece of mind. Shakesphere once said kill all the layers