FMPEG is a command-line tool for working with audio and video, do questions relating to it belong more on Audio-Video production?
I think FFmpeg questions should be steered / migrated to VP.SE.
The trouble, IMHO, with @slhck 's line of thinking is that SU has a very broad scope; it's a catch-all site. His policy would be OK for sites with a narrower focus like, say, Home Theater. But if questions traditionally asked on Super User, for lack of a dedicated site, are allowed to remain on SU even after a dedicated site is introduced, then that leads to a vicious cycle where new visitors searching for a forum to ask FFmpeg questions continue to post on SU. This simply tends to make the new site stillborn. Especially in the case of VP where most questions which are posted there deal with the use of software such as editors, compositors and converters such as FFmpeg.
Note that Video Production is not a site for video experts only - it is a site for any questions, asked by anyone, concerning methods, processes and tools related to video generation or modification.
The sentiment about loyalty should be loosely applied within a community of sibling sites, especially for a catch-all site such as SU. If a team leader continues to do a task, even after a job has been created for a task, on the basis that their job is to oversee everything, then that job officer will remain idle. The idea of a dedicated subsite is to concentrate activity and people with specific interest/expertise at a single locus, for a more efficient operation, resource-building effort and attracting and retaining qualified users in that domain.
ffmpeg is computer software. It's not exclusively used by programmers or audio-video experts—it fits perfectly within the scope of Super User. That is not to say questions about ffmpeg are off topic for Audio-Video Production, but one of the most important rules regarding migration is:
Don't migrate on-topic questions away.
You should be a little jealous of your community. Don't reject good, on-topic questions and move them to another site. The whole idea is summarized in this blog article: Respect the community – your own, and others’:
As members of a community, your first loyalty should be to that community. When evaluating a question, you shouldn’t be looking to push it off on some other site; instead, ask if it could be appropriate and on-topic for you, the experts who the author decided to ask.
Furthermore, it is not allowed to migrate questions to Beta sites (which AVP.SE is). These sites are still growing, and they should be able to attract traffic by themselves—not by force-feeding them with questions from established sites.