Video and audio travel in the same stream but take different routes through the device. Video is decoded and drawn. Audio is either decoded to PCM by the app or passed through as a compressed bitstream to whatever is downstream, a television, a soundbar or a receiver. Fire TV apps that support Dolby audio are expected to check whether surround output is available and to fall back to a non-Dolby track when it is not. A player that skips that check hands an AC3 bitstream to an output that will not take it, and the result is exactly what you see: full picture, complete silence, no error message at all.

Three questions, in this order. Is it one channel, one category, or every channel including video-on-demand? A single silent channel means that feed's audio track differs from its neighbors, usually a Dolby track where the others carry AAC. A whole category silent points at how that group is encoded upstream. Everything silent, including menu clicks and other apps, is the device or the HDMI path, not the service at all. Test a second app on the same stick as a control. If a completely unrelated app also plays silently, stop adjusting the player and look at Fire TV's own output settings.

Two, and they are worth setting deliberately rather than by trial. First, the Dolby output mode under display and sound: automatic negotiation works when everything downstream reports its capabilities honestly, and older televisions and cheap HDMI switches often do not. Forcing stereo output makes the device decode centrally and hand simple PCM to the television, which trades surround for reliability and fixes a large share of silent-channel reports. Second, surround passthrough: if your set accepts Dolby Digital but not Dolby Digital Plus, most Fire TV devices can convert down and pass it along, but only where the downstream device reports support correctly.

Then the audio track is the variable, and most players expose it. Open the track selector during playback and see what is listed: several channels ship more than one audio track, and the default is not always the one your setup can handle. Selecting a stereo or AAC track where one exists produces sound instantly and tells you the codec was the cause. If a channel offers only a Dolby track and your path cannot take it, switching the whole device to stereo output is the fix rather than a workaround. Note which channels needed it, because that pattern is useful information for support.

Sometimes the feed simply arrives without usable audio, and no setting on your side creates sound that was never sent. You can confirm this quickly: if the same channel is silent on a phone, on a second player and on a different network, the fault is upstream. That is worth reporting with the channel name rather than absorbing. A service running 54,000+ live channels will occasionally have a source with a broken audio track, and what matters is how fast it gets corrected. IP4KTV publishes 99.99% uptime, roughly 53 minutes a year, which is the kind of figure worth asking any service to state.