One night about 4 years ago I came out of a night club with a really loud ringing my ears and a muffled feeling. It wasn't the normal ringing after such a night out. I wasn't a big one for the clubs and remember noticing it was uncomfortably loud in there. Louder than seemed normal. I did notice they had positioned the 4 foot high speakers next to the dance floor (not that I was busting out shapes of course ha).
It lasted a month then died down. I still have a permanant quiet tinnitus but I don't notice it really as your brain trains itself to overlook the sound I think. If I concentrate on it then it's there but it never causes problems. It is a sign of permanent hearing damage. I no longer go to clubs and all that malarky. At the time I held a class 1. My recent class 1 renewal included an audiogram which showed very good hearing. You're allowed a natural decrease so I should be ok but I watch my hearing now. I know quite a few collegues who have high tone loss due to guitars,music or palying drums. You have to be careful with it.