Edit 2020:
Homebrew now installs libvorbis by default, so you can just run:
brew install ffmpeg
If you’re using Homebrew, try this:
brew install ffmpeg --with-libvorbis
…or, if you’ve already tried installing ffmpeg with Homebrew…
brew reinstall ffmpeg --with-libvorbis
If you want libvpx, too:
brew reinstall ffmpeg --with-libvpx --with-libvorbis