I think Chromium already does sandbox ffmpeg in the renderer process because of their "Rule of Two": https://chromium.googlesource.com/chromium/src/+/HEAD/docs/s...
Thus:
1. Code which processes untrusted input
2. Code written in unsafe languages like C or C++
3. Code that runs without a sandbox
So ffmpeg should be sandboxed, same as the network code and GPU process are sandboxed.