Currently "libvpx" does not build on M1 macs. Here's how to build ffmpeg without this dependency (until it's fixed). We need to create a local port and build it (it's not strictly necessary to add this port tree to /opt/local/etc/macports/sources.conf)
sudo mkdir -p /opt/local_ports/multimedia/
sudo cp -r /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/ports/multimedia/ffmpeg /opt/local_ports/multimedia