I have a fix for that!!!! It appears to started after the last update to iOS 10.3.2. All of a sudden people started complaining that I sounded muffled. At first it was the same person, so I thought it must be on their end. Then the complaints came in from other people. There is a setting that can be changed that will fix this issue!
Go into your Settings on your iPhone. Scroll down to the General button and open it.
While you are in the Settings–>General area, scroll down to Accessibility.
Under Settings –>General–>Accessibility look for Hearing Devices.
Go into Hearing Devices.
Settings –> General –>Accessibility–>Hearing Device
As weird as this is about to sound, turn ON Hearing Aid Mode. Yep, you read that correctly!!!! Make a test phone call to someone who complained about you being muffled and see if this has indeed fixed your issue! Let me know how it works for you!