Non-free version of the ffmpeg package for yiffOS not included in the official repos for licensing issues.
Add https://uwueviee.live/yiffos-repo/x86_64/knot/ to your repo list along with your mirror list and then run bulge i ffmpeg-nonfree.
Modified version of https://gitlab.com/yiffos/packaging/packages/-/blob/main/ffmpeg/PKGSCRIPT