Few things you can try, first off make sure you are properly powering the device along with making sure its is updated.
If you are experiencing this problem, please make sure that your firmware is up to date. We know that using the TV USB port to power the Fire TV will drop frame rates which can cause audio sync issues so please use the main power supply.
Home Theater Audio is Out of Sync
Turn on your Fire TV device.
Select Settings from the home screen.
Select Display & Settings.
Select Home Theater Settings.
Select Lip Sync Tuning.
Follow the on-screen instructions to synchronize your audio and video.
If using a sound bar / surround sound.
First, if you are running your sound through a sound bar or sound system, turn the TV speakers OFF via the Vizio remote, Menu > Audio. Also in the Audio menu, find the Lip Sync adjustment. It will probably be on 0.0 with settings from 0.0-5.0. Put the setting at 2.5, save your settings and exit.
How is your device wired to your TV is it directly connected or routing through anther device?