Install FFmpeg with homebrew. You'll need to install it with a couple flags for webm and the AAC audio codec.
brew install ffmpeg --with-libvpx --with-libvorbis --with-fdk-aac --with-opus| /* WordPress Gallery | |
| ------------------------------------------ */ | |
| .gallery { | |
| margin-left: -0.5em; | |
| margin-right: -0.5em; | |
| } | |
| .gallery:after { | |
| display: table; |