You can easily still hear people when using them, not at all when having music blasting in your ears.My opinion is that from an HS&E perspective you wouldn't tell a guy not to use ear plugs if he feels the need to wear ear plugs. I am no lawyer but that's not a position I'd like to be in if he comes back and said my shop made him deaf.
So if earplugs are carte blanche you have no leg to stand on banning headphones/ear buds. They are both in or they are both out.
I wear earplugs all the time in the shop, hearing protection when running tractors.