No sound or sound out of sync

Try these one at a time. After each step, check if it's fixed.

Check the basics

Make sure the TV isn't muted and the volume is up. Try another channel: if only one channel has the problem, it's likely that channel. Tell us which one.
Restart the app and device

Unplug your Fire TV or TV box from the power for 30 seconds, then plug it back in. While it restarts, also restart your modem/router the same way. This clears up most problems.
Turn off audio passthrough

If you use a soundbar or receiver, go to Settings → Playback in our app and turn audio passthrough off. This fixes most "no sound" problems on soundbars.
Switch the audio decoder

In Settings → Playback, change the audio decoder setting (hardware ↔ software) and try again.
Fix out-of-sync sound

Change channels and come back, which re-syncs the stream. If it keeps drifting, also check your TV or soundbar's own AV sync / lip sync setting.

Still not working?

Send us a message and we'll look into it. We'll fill in the steps you've already tried so you don't have to repeat yourself.