For HLS audio-only outputs the codecs supported are AAC or Dolby Digital (AC3) : https://docs.aws.amazon.com/mediaconvert/latest/ug/supported-codecs-containers-audio-only.html.
You will need to select "Container for audio-only output" in the Audio section of the Output Settings for the HLS output group : https://docs.aws.amazon.com/mediaconvert/latest/ug/setting-up-audio-only.html.
Depending on the scenario you can also select "audio only variant stream" in the Audio Track Type on the same section.
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