Although the iPhone 7/7 Plus is waterproof to a degree, water can still get inside the speaker. It can’t get out if this happens because of the mesh design behind the speaker holes that’s there to keep water out – but also keeps water in! As a result the sound is muffled or just generally distorted.
Thanks to one canny Redditor, one possible fix that could work for any iPhone with water in its speaker is to use a tone generator app to force the water out. Tone generators create basic pure musical notes and essentially you’re attempting to vibrate the water out using sound waves.
Just install the Sonic app and then set it to make a low frequency noise around the 165Hz range, although be prepared to lower or raise the frequency to get the best effects (just swipe up or down on the screen to change the frequency). Hopefully you’ll see the water start to come out as bubbles or a film of water across the speaker grille. Use something like rolled tissue paper or q-tips to CAREFULLY dab it up.