Noise Induced Hearing Loss (NIHL) is insiduous, gradual and incurable.
Once it's gone that's it, it's gone.
In the UK, the Control of Noise at Work Regulations 2005 were introduced in early 2006 and significantly reduced maximum exposure levels, which apply not only to employees (flight, cabin and ground crew) but also to visitors (passengers) to company premises (aircraft).
Any exposure exceeding these limits could result in NIHL.
However, on short - medium haul flights it should not be a significant issue as time weighted averages would probably not exceed maximum exposure levels but long haul could be different, especially for the more frequent flyers, i.e. long haul flight and cabin crew.