OPUS audio fails to render when using avformat demuxer

When using the avformat demuxer to play MKV media with an OPUS audio track, any audio output will be silent. The problem is not reproducible when using VLC's own MKV demuxer.

Debug log: https://pastebin.com/76AH2JXD

This is reproducible with any MKV file with an OPUS audio track.

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