LO 5.4.2-3, 1:5.4.2-3~bpo9+1 to be specific, on Debian Linux 9.x stretch. Similarly for Debian 8.x “jessie”. Any attempt to load a video or audio file results in “format of the selected file is not supported”.
Also, I took a presentation created on an older version of LO. It does not play on 5.4.2. So it appears there has been a regression.
You will need youtube-dl or equivalent to copy them to your computer.
Addendum: I pulled in the LO packages LibreOffice_5.4.3_Linux_x86-64_deb_helppack_en-US.tar.gz LibreOffice_5.4.3_Linux_x86-64_deb.tar.gz and installed those. Apparently they can live comfortably along side the debian packages (different locations). Anyway, that can play the older presentation. However, when I try to import the same embedded video into a new presentation, it imports, but the video jumps to different parts of the frame.
OK, I solved the jumping around problem: set the size of the video to less than the size of the slide show.
With that fix, this may solve the problem.