Sorry to keep bothering you on this, I am just tearing my hair out for hours now and I dont get it to work. There was no 'startup' folder in my Videos folder so I created it and placed the file called startup.mp4 in there. I have also placed it in the root of the Videos folder. This doesn't work. i keep getting the original BigBox start screen and sound. Now, in the Videos folder I can see other folders called Arcade, Nintendo 64 etc which contain some videos. However the root of my Launchbox/Videos folder is empty (apart from startup.mp4) isn't that a bit weird ? I think my solution lies in locating the original startup screen but I cannot find it anywhere.
This is something else I discovered. If I overwrite 'startup' in the sounds folder and then start BigBox, the Startup.wav gets recreated. Could that be a clue ?