I've been having people complain they cannot hear me or that I sound extremely muffled when I making phone calls on my Moto X4. I confirmed it was definitely the phone because I tested it with a wired headphones/microphone without issues.
Many forums suggest returning the device as a RMA. This really isn't an option for my nearly 2 year old phone which is out of warranty. One interesting comment I read was related to the position of the microphone holes -- one of which is very near the fingerprint reader. My phone is always in a case, so the rear microphone hole seemed clear but the front microphone hole seemed slightly dirty.
The phone is rated IP68 which allows for submersion in water for 1 hour up to 1 meter. Since my next option was to buy a new phone, I decided to put the IP68 rating to the test.
Of course, what I did next -- only do at your own risk. I am not responsible for anything you decide to do to your own phone.
With 100% rubbing alcohol, I cleaned out the microphone holes with a cotton swab and let dry. Using a recording app, the sounds was marginally better. More drops of alcohol but this time I used an electric toothbrush to vibrate all the junk which was more than I expected (ick).
Voila, things sounded 100% better and I no longer need a new phone!