I used the apple call back service and he said they werent aware of any issues and that some people in the forums where say the i5 louder...
I also went into a an apple store and as you know theres nothing genius about them infact dimwit came to mind... All he offered was a remanufactured phone. I tested some other i5s and they were the same..
Im not convinced its the EU limiter as this is a "recommended" limit volume which can switched off.. There is definatly new sound signature, i use the sennheiser ie 80s which have a base adjuster, for the i4s its on max, i5 minimum.. My B&Ws sound awful and muddy with i5 but great with i4s.
I was hoping ios 6.1 would bring a fix but sadly not..
The biggest reason for me owning the iphone series was for music quality and given i can no longer enjoy my music its time to look elsewere..
I also have lumia 920, and with the various software improvements it actualy sounsd impresive far better than my I5..
There are MANY other benefits , both nokia and microsoft seem to be trying very hard and are making good progress...