I am on my 4th or 5th 4S and everyone has lost speaker volume. This one is less than a month old. Earbuds work fine. Volume controls work; the phone speaker gradually loses volume. Any help out there?

Apple 4s Iphone. This is my 4th or 5th phone in less than 2 years due to declining speaker volume. Controls work; earbud volume is high, so it is something other than the speaker failing. This phone is less than a month old still in warranty, but I want to get this resolved or go to another manufacturer, although I am on Apple desk top and lap top. Anyone have this problem. The only thing I do different than most is I use a Mopie. My wife has the same phone, a 4S with no Mopie and she had never had this problem.