FFmpeg is a software library for reading and writing a wide range of image, movie and audio files.
Use of FFmpeg
Usage of FFmpeg is granted by the LGPL version 3 license, available for viewing here, as well as in the Touch folder in TouchDesigner's installation directory.
Builds of TouchDesigner099 2016.4180 and later
The source is the modified ffmpeg source from the release/3.1 branch. It can be downloaded from here. The branch is release/3.1.Derivative. The exact commit hash is 650671b908.
Builds of TouchDesigner099 2016.4160 and earlier
The source is the unmodified ffmpeg source from the release/2.8 branch, commit hash 913c642c21. You can get the source here.