Q: Iphone 7 sounds muffled

I recommend following these steps:

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

especially the one about contacting Apple Support if nothing else works.  Or make a reservation at a Genius Bar at an Apple Store near you:

http://www.apple.com/retail/geniusbar/

If you have been testing out the new Waterproof feature, water can get into the speaker ports. simply let the device dry out overnight and try again.

Who is your carrier? There is a known problem related to Verizon's network at the moment. Verizon and Apple are working on a solution.

Perhaps it is a carrier problem. My Iphone 6 has the same issue on Verizon, but I have both another Iphone 6 and an Iphone 5s that work fine.

I had this problem.  Having researched on the web and contacting apple support I tried all of the other suggestions you have received.

The problem is the microphone on the back of the phone to reduce backgound noise.  I switched this off and everyone says that I sound clear (as if I'm standing next to them).

To switch off - settings / general / accessibility - scan down to hearing section and switch off phone noise cancellation.