IPhone 6s Plus muffled voice

everyone i talk to say my voice is muffled. i can hear them but i have to repeat everything i say. i went to the Apple store when i first bought it and they replaced it. my voice is just as muffled. i searched the internet and nothing helped. i went to the att store and they couldn't help me. i thought maybe it was something in settings but when i went to the att store they said no that wouldn't cause it. Other then buying something besides APPLE do you have any idea what i can do. i hardly talk on it because it's too hard. not worth it. i had thought about just getting a little flip top just so i can call people. I thought i would try one more time. Any thoughts?