audio – How to use FFmpeg to modify metadata downmix levels of AC3 file

This is totally new to me, so please bear with me 🙂

I’m trying to use FFmpeg to modify a metadata called “downmix levels” of an existing AC3 file (Dolby Digital surround sound file)

I have tried with at least a million different combinations of the commands, like this one
ffmpeg -i input.ac3 -metadata -center_mixlev=0.707 -c ac3 output.ac3

The FFmpeg tool does create a new AC3 file, but the metadata bsi field is still encoded to exactly the same values as the input file

The metadata is described in the link below:
underpop.online.fr/f/ffmpeg/help/ac_002d3-metadata.htm.gz

Any help is greatly appreciated