Why can't I hear out of my headphones?

If the headphones have been tested, then the next step is the Lightning port. It’s relatively easy to try a new Lightning port, you don’t even have to remove the old one to test it. Just disconnect the old one (battery first!), then overlay the replacement one, reconnect the battery and then test the headphones again. Check out this guide for details on how to open up the phone and (eventually) replace the Lightning Port completely if necessary.

If that doesn’t solve you’re problem, then a logic board issue is the likely culprit, in which case you’d want to have this looked at by a micro-solderer. The iPhone 7 is prone to audio related issues caused by a weak copper trace on the logic board.