I tested it against a 4S, and it seems like the 5 is a little quieter, don't know what to do really as I don't think it is quiet enough to warrant a replacement. Need to find someone else with an iPhone 5 and test that against mine.
Edit: I just called Carphone Warehouse who were acting on behalf of O2 and the guy on the phone said he was having the same problem with his, it seemed quiter than his 4S, however he doesn't believe this to be a fault. He has advised that I call back in a couple of days to see if they have anymore information.
I think I'm just being pety to be honest, it really can't be much of a difference.