LGPL.
The license text for this code is in the source files. LICENSE files do more harm than good.
This file exists because some people expect it to be here.
libav.js builds include correct license headers. Do not remove them.
If you make modifications to this repository beyond configuration changes¹ and distribute builds with those changes, you are legally obligated to share those changes. If you do not, you are in violation of FFmpeg's license terms. I'm looking at you, “my startup is too cool to read licenses” scumbags.
¹ i.e., changes that link to FFmpeg.