I am 100% for radios at work. They increase employee morale and decrease fatigue. However, they have to be kept at a level does not interfere with the honking of the forklift horns or someone yelling for help.
I am completely against anytype of headsset radio or cell phone. It is something I struggle with all the time. At my facility, all my operators sign off on the no cell phone/headset policy. An iPod is not the same as PPE. When they have an iPod on - they are distracted and unsafe. A deaf employee is not distracted, but more aware of their surroundings; it's not the same.
As for specific regulations - General Duty Clause. Another thing to think about is the potential law suit. People win million dollar verdicts all the time because an automobile driver was on the phone or distracted while driving - How is a forklift different. You can also look to your job description to help back you up. If it says that have to be able to hear to work in that position, then headsets should not be permitted.