MP4 file format appears to support a default flag on audio tracks as well as subtitle tracks, although not all MP4 players respect this setting, many simply pick the track with the highest ID.

To ensure maximum compatibility, it would be nice if MP4Box could allow setting this flag. FFMPEG currently allows it with the `-disposition:a:1 default` flag, so that may be a useful reference for how it's being done.

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

FR: Allow setting default audio track with MP4Box
This is a GitHub Issue
repo:gpac
owner:gpac
Title : FR: Allow setting default audio track with MP4Box
Issue date:

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MP4 file format appears to support a default flag on audio tracks as well as subtitle tracks, although not all MP4 players respect this setting, many simply pick the track with the highest ID.

To ensure maximum compatibility, it would be nice if MP4Box could allow setting this flag. FFMPEG currently allows it with the `-disposition:a:1 default` flag, so that may be a useful reference for how it's being done.

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

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