Is hearing protection required as part of Arc Flash PPE? If so what are the requirements.

Yes - From the 2015 Edition of NFPA 70E:

130.7(C)(5) Hearing Protection. Employees shall wear hearing protection whenever working within the arc flash boundary.

The sound is related to arc blast. So far the test work has been inconsistent at best, so there is no "dB rating" or anything like that. The standard doesn't call for arc rated ear plugs but Hugh Hoagland's group has done some preliminary testing. My own feeling is that the ear plugs are going to be experiencing the same incident energy as the side of the head so it might not make sense to require a 40 cal/cm2 ear plug that is worn under a 40 cal/cm2 arc flash hood but it may have some merit for an ear plug that is worn with an arc rated shirt and pants only for 1.2 - 4 cal/cm2 work.

Hugh can chime in himself, but I believe I remember his research concluding that standard ear plugs are just as effective for the practical effects of an arc flash as specialized ear plugs would be.

Note that because of the equipment already being worn in an arc flash situation (balaclava, hard hat, face shield) that ear muffs would not work as a solution.

I can't recall but I would not be surprised if there are problems with the foam padding on the ear muffs melting since as I recall the only ear plugs that were possibly an issue were the foam ones that you roll between your fingers before inserting. Almost all the others performed very well with regards to incident energy.