Some time ago i had the exact same problem, every other video not playing when i move to another Game, only sound.
I switched to from VLC to Windows Media Player only to realize, that without a codec pack Media Player could not play Steam Trailers in webm format.
So i converted them all to mp4 und my problem was gone.
With the latest Launchbox Update i installed the recommended codec pack so windows media player now can play those Steam webm videos.
After importing some Steam Games i left them in webm format und you know what? The problem was here again.
To be clear, It does not only affect playing webm files but also playing mp4 videos.
So i converted again those few webm files to mp4 and the problem was again gone.
I wonder if only the presence of webm files in any Launchbox Video folder could possibly trigger this strange behaviour
So in short, if you have any Steam Trailers in your Launchbox folder, try moving them out and test if other mp4 files now play normaly in Launchbox.