6 ch audio not playing in web client

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AAC ≠ AC-3 (DD), so I'm wondering if you could provide the full mediainfo from either Jellyfin or terminal. Additionally, when you select the max number of channels, does that trigger transcoding? If so, you should find an ffmpeg log in Dashboard > Logs that can be posted to sourcebin or pastebin that we can look at. Including your Jellyfin log would be a good idea too. Server version, host OS, type of install might be relevant as well.