Flex supports stereo and 5.1 input.
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If a user selects 5.1 as their audio input in the Presenter app we will send 5.1 to the Apple TV, and downmix for all other devices.
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To downmix from 5.1 surround sound to stereo, we use the matrix recommended in RFC 7845 (which defines Ogg encapsulation for the Opus codec, and is also what the WebRTC ‘multiopus’ implementation is based on). This matrix appears to yield better results than the LtRt or LoRo matrices defined in the ATSC standard for digital audio compression, largely because it does not discard the LFE channel and has less impact on the dynamic range of the audio signal.

5.1 surround sound is only available to users with a Pro subscription or using a Weekly or Monthly Pro credit.