Distorted sound of VO on iphone 5s

Hello I have just purchased my new iphone with great enthusiasm only to be dissapointed by really bad quality of sound on VO. It is new iphone 5s, I have tried abslutely everything! I went to the apple store and was told that perhaps my handset is faulty and was offered a replacement. But after trying 4 other new handsets with the same problem I was told that this is probably the problem with the batch that they:Apple store received. I was also told that that they have send their comment and I should hear within 24 hours instruction about how to fix this. When I did not hear after 24h I contacted the apple support line and was offered 3 options of returning the phone to the apple store. I have chosen to make an appointment again in the Apple store and apparently problem should b fixed?! Really feeling sceptical as majority of staff really have no experience and expertise of Accessibility and dealing with customers who are blind. I would like to add that they all have been trying to be very helpful and polite. Very, very frustrating especially when you rely so much on the VO function that help you to be more independent, therefore it is so important that the sound quality needs to absolutely spot on. I am a bit lost with this issue and would appreciate any helpful advice or solution.