Apple India — I phone 6 & 6s

Address: Apple India Ltd, Faridabad, Haryana, 121002

Sir,
In apple i phone 6 & 6s, there is one manufacturing defect that sound becomes slow after some time, i have two phones at my home, same problem in that & i have many collegues in my university with the same model, facing the same problem, kindly provide the solution.
I think the company should refund our money back or replace the phone with some other model.