trying to set up the cube with my soundbar over hdmi arc and there's no sound, just video.

Diagnosis

This issue is almost always caused by a failure in the HDMI-CEC (Consumer Electronics Control) handshake, specifically regarding the "Audio Return Channel" (ARC) protocol. In a standard setup, the Fire TV Cube 3rd Gen sends video to the TV, and the TV "returns" the audio signal back down the same cable to the soundbar. If the TV is not configured to output audio via the ARC port, or if the Cube is attempting to send a high-bitrate audio codec (like Dolby Atmos or DTS:X) that your specific soundbar or TV cannot decode, the system will default to silence to prevent hardware damage. You can confirm this is a handshake issue if you can hear audio through the TV's built-in speakers but not the soundbar, or if the soundbar displays "No Signal" while the video is playing. This often happens because the Cube 3rd Gen defaults to "Best Available" audio, which may exceed the bandwidth capabilities of an older HDMI 2.0 cable or a TV that doesn't fully support eARC (Enhanced Audio Return Channel), leading to a complete audio dropout rather than a downsampled signal.

How to Fix It

Follow these exact steps to force the audio handshake and synchronize your hardware:

1. Physical Verification: Ensure the HDMI cable is plugged into the specific port on your TV labeled "HDMI (ARC)" or "HDMI (eARC)". Using a standard HDMI port will result in zero sound. Use a High-Speed HDMI cable (Category 2) or an Ultra High Speed cable to ensure 18Gbps bandwidth.
2. TV System Settings: Using your TV remote, navigate to the Audio/Sound menu. Change the "Digital Output Format" from "Auto" to "Pass-Through" or "Bitstream". Ensure "HDMI-CEC" (which may be called Anynet+ on Samsung, Bravia Sync on Sony, or SimHlink on LG) is toggled to ON.
3. Cube Audio Configuration: On the Fire TV Cube, navigate to Settings > Display & Sounds > Audio > HDMI Audio. Change this from "Best Available" to " surround" or "Stereo" temporarily to see if sound returns. If it does, you can gradually move back up to "Dolby Digital Plus".
4. Equipment Control Reset: Go to Settings > Equipment Control > Manage Equipment > TV. Select "Change TV" and run the setup wizard again. This forces the Cube to send a new CEC command to the TV to request audio routing to the ARC port.
5. Power Cycle Handshake: Unplug the power cables from the Cube, the TV, and the Soundbar. Wait 60 seconds. Plug in the TV first, then the soundbar, and finally the Cube. This clears the HDMI cache and forces a fresh handshake.

If That Didn't Work

• Bypass the TV via Optical: If your TV's ARC implementation is buggy, connect the Fire TV Cube to the TV via HDMI, but run a Toslink Optical cable from the TV's "Optical Out" directly to the soundbar. This bypasses the HDMI-CEC handshake entirely and is the most stable connection for older soundbars.
• Direct Soundbar Connection: If your soundbar has an "HDMI IN" port in addition to "HDMI OUT (ARC)", plug the Fire TV Cube directly into the soundbar's "HDMI IN" port. Then, connect the soundbar's "HDMI OUT" to the TV's ARC port. This allows the soundbar to strip the audio signal before passing the video to the TV.
• Disable Volume Control: In Settings > Equipment Control**, try changing the "Volume Control" method from "CEC" to "IR". Sometimes the CEC command for volume interferes with the audio stream initialization.
• Update Firmware: Ensure the Cube is on the latest version by going to Settings > My Fire TV > About > Check for Updates. A known bug in early 3rd Gen firmware caused intermittent ARC drops with certain LG and Sony OLED sets.