Setting default audio tracks

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Some clients (e.g., Roku) allow you to set this in their main settings, also in the Jellyfin profile Settings for your account, but I have found that a mixed bag as far as whether clients actually respect those settings.

Surefire way to set default audio? At the media level. This is actually set at the stream level and can be done pretty easily, but is difficult to do at a batch level (or I haven't figured out a way to do it yet). Mkvtoolnnix can set default track metadata, as can several other tools.