I spent ages trying to figure out why I couldn't play mp4 videos when exporting to NWjs.
Turns out NWjs won't let you play mp4 videos by default (pretty sure this is for legal reasons). You can fix this by replace the default "ffmpegsumo.dll" file in your projects windows 32bit directory (pretty sure 64 bit is also available somewhere out there!).
I've only tested this for windows versions.
Here's where I found the file: http://stackoverflow.com/questions/29590898/where-to-find-ffmpegsumo-libraries-for-nw-js-0-12-0-play-audio-files-in-nodewe
NOTE: This is only for personal use. Look into the legalities of using mp4 codecs before publishing a game with this file.