What does OSHA say about the use of ear buds in a manufacturing environment?
We have several questions and responses in our archives about using ear buds and/or ear protection, in work environments. Some of them have many links to articles. You may find it useful to go to categories and check out Music and Noise in the Workplace.
If you are asking about reducing harm from noise in the work environment, your best resource is the OSHA website. They don’t have rules about whether or not ear protection is worn, but they have advice about protecting hearing.