Some customers have reported getting some audio sync issues with Fire TV where the words they hear do not align with the video (usually audio is ahead of the video). This is with watching over-the-air antenna channels, YouTube TV, Sling and other apps. While there is not one thing that always fixes the issue, as the solution varies depending on the TV you are using as well as whether you have a soundbar or an AV system, changing these settings seems to help most customers.
To change some settings that should help with audio-sync issues, on the Fire Stick, go to:
Settings > Display & Sounds > Audio > Surround Sound > try "Stereo" or "PCM"
Then go to: Advanced Audio > Turn Volume Leveler to OFF
Also: Turn Dialogue Enhancer to OFF
Next, press back button until you get back to Settings
Go to My Fire TV > Restart > Restart and let your Fire TV restart.
If that does not help, then you will need to look in your TV, soundbar, or AV system settings (using the remote that came with those devices) and look for an audio delay setting and adjust until the audio seems to line up. If you are unsure if your TV has these settings, contact the manufacturer of your TV, soundbar, or AV system for advice.