Assuming you're after SQF standards as well as law since you're posing this in a SQF subsection:
11.3.3 Clothing and Personal Effects
11.3.3.8 Jewelry and other loose objects shall not be worn or taken into a food handling or processing operation or into any area where food is exposed. Wearing plain bands with no stones, prescribed medical alert bracelets, or jewelry accepted for religious or cultural reasons can be permitted, provided these items are properly covered and do not pose a food safety risk.
11.3.4.2 All visitors, including management staff, shall be required to remove jewelry and other loose objects in accordance with the facilities Good Manufacturing Practices and 11.3.3.8. All visitors shall wear suitable clothing and footwear when entering any food processing and handling area.
11.3.5.4 Provision shall be made for staff to store their street clothing and personal items separate from clean uniforms, food contact zones, food, and packaging storage areas.
From US Federal law:
PART 117—CURRENT GOOD MANUFACTURING PRACTICE, HAZARD ANALYSIS, AND RISK–BASED PREVENTIVE CONTROLS FOR HUMAN FOOD
Subpart B—Current Good Manufacturing Practice
§ 117.10 Personnel.
The management of the establishment must take reasonable measures and precautions to ensure the following:
(7) Storing clothing or other personal belongings in areas other than where food is exposed or where equipment or utensils are washed.
These selections don't even begin to address the OSHA safety concerns about employees in a manufacturing environment. Not being able to hear equipment approaching or other employees in distress is a problem. Not to mention liability of an employee who listens to music at ear damaging decibels coming back to sue your company for hearing loss ("I had to turn my earbuds all the way up to hear music because your plant has loud noise!").
If employee happiness is truly such a big concern that music must be played, the only good idea I can present is music over a PA system at an appropriate level. Any speakers you add must be addressed in your sanitation program, glass and bp program, and sanitary installation guidelines. Generally speaking, even a boombox is a personal item that cannot be allowed (because how often do your employees wash their boomboxes, then they want to touch it during the shift when they should be on the line working with sanitary hands, etc), so if you have to cave and provide some version of radio, that has to be a company product accounted for in all of your programs.
In short, tell 'em music is against code and against Federal law, and help find other ways to improve morale. The beatings shall continue until morale improves! 
) One of them says their priority is keeping the employees happy above everything else LOL which causes the employees to think they can do whatever they want sometimes. Which is great you know to want to keep employees happy, but not on top of food safety or other more important things.
