Welcome to Android Central! Is it also silent on wired or Bluetooth headphones?
Try booting into Safe Mode, which temporarily disables all 3rd party apps.
https://support.t-mobile.com/docs/DOC-11724
If the problem disappears in Safe Mode, then something you installed is causing the problem. You may have to uninstall apps one by one until the problem disappears. If the problem persists in Safe Mode, then it's more likely to be a hardware or firmware problem.