The main reason for this beta release is to adjust the Audio and Video plugins to work better with the new autoplay restrictions in Chrome 66 (due out this week). Browsers are tightening the restrictions and we have updated our code to comply with them. You may find media cannot autoplay in a browser window until you interact with it, which can also affect previews. This is part of the new restrictions - simply click, touch or press a keyboard key and playback should begin.
There are also a couple of new NW.js features which should help with opening included project files. Note these features are not yet in C3, but will be added in the next beta, so if you use them you'll temporarily be unable to import the projects to C3 until the next beta. Finally as ever there's a few more bug fixes. All being well, we hope to make a stable release in the next week or two.