Did you replace the ffmpeg library that came with your new NWjs with a pre-compiled version that supports modern video formats? What you need to do is to download the appropriate ffmpeg.dll (ffmpeg.so if you use Linux) from the site associated with the link below and use it to replace the one that comes with your new NWjs wherever that is (game folder for Windows and swiftshader directory for Linux).
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And replace the one that comes with NWjs with the prebuilt library you download from the above. The one that comes with NWjs doesn't handle some of the new video versions and you need to replace it with one that does.