My assumption is, if you are dealing with video, you will need to replace the FFMPEG dll that ships with NW.js. Google pays to license commercial video/audio codecs for Chrome. However, they don't do that for Chromium. So it only has the open source audio/video codecs. NW.js is based on Chromium.
NW.js has a page about this:
It includes instructions on how to do a custom build of FFMPEG to be compatible with your version of NW.js. It also links to prebuilt binaries maintained by the community:
Though these are not available for all version of NW.js, so you'd need to pick a version that matches, and also make sure your global Node version matches the version in NW.js.
Also, you'd still need to deal with licensing these codecs for your software if you are concerned with the legal usage of them.