No videos and no audio
Currently, no videos can be embedded in z-Tree if z-Tree unleashed is used. This is due to Windows Media Player not being available on Linux.
Secondly, RFB (the network protocol behind noVNC, and therefore, z-Tree unleashed) is really ill-equipped to stream videos. Even if another method of opening a video would be used (such as with VLC, mplayer or mpv), the video would appear very choppy.
However, even if that were resolved, an even more pressing limitation is that audio is not available. Audio cannot be streamed over RFB. Therefore, no solution will ever be forthcoming (at least not in the current configuration of z-Tree unleashed).
POSSIBLE WORKAROUND: Upload the videos on YouTube (possibly using the “Not Listed” function) and tell subjects to enter the video’s URL in a separate browser tab when it is prudent. You might wish to use a URL shortening service to make the URL more user-friendly. Or you can send subjects the link to the video in the email with the participation link, if your experiment allows that.