ClearView Flex supports 5.1 output for viewers using the Apple TV app. All other viewers will receive a downmix as described in the ClearView Flex 5.1 downmix specs. The input to the Flexbox encoder should be in standard SMPTE / TV 5.1 format (L R C LFE Ls Rs).

Your session presenter also has to enable 5.1 input via the Flexbox input/output settings panel. This can be accessed during an active presenter session from the cog icon at the bottom right of your stream viewer.

Viewer AppleTV requirements:

Currently, uncompressed LPCM output is supported, as well as bitstream formats such as AC3 (Dolby Digital 5.1).

Ideally, your surround sound receiver either needs to be in-line between the AppleTV and the display (HDMI from Apple TV to receiver, HDMI from receiver to TV), or if you have a newer TV and receiver, connected directly to your display using HDMI eARC (Enhanced Audio Return Channel).

You can also use S/PDIF (also known as Digital Coaxial, TOSLINK or Optical) and HDMI ARC connections but as these can only carry 2 channels uncompressed or compressed surround bitstream formats, there may be some quality degradation.

Additionally, some hardware such as earlier Sonos soundbars do not support 6 channels uncompressed LPCM and thus you will be required to use Dolby Digital 5.1.
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You may also validate that in the AppleTV audio settings, "Change Format" should be set to "Off" for most uses (and there should be a message in the bottom left stating output will be uncompressed multichannel LPCM as below), and can be set to "Dolby Digital 5.1" for users with HDMI ARC or Optical configurations, or receivers/soundbars that do not support surround LPCM. Dolby Atmos and other formats such as 7.1 should be disabled.

If you require Optical output for your receiver/soundbar, it is best to use a hardware HDMI audio extractor between the Apple TV and your display, rather than using the optical out from your TV. There are many models available, but an example of one would be: Audio Stereo Extractor Converter