The audio for my amv files is of type adpcm_ima_amv.
It only has decoding support, not encoding.
I've found an open source project that implemented this encoder, but it doesn't work with the new version of ffmpeg.
Since I'm not a C developer, I need help merging it ffmpeg.
Please take a look at the issue I posted with more details.
I can maybe promote someone who helps me.
Here is the ticket I've opened: https://ffmpeg.org/trac/ffmpeg/ticket/747