Hey guys, big GameEx fan here, thanks very much Tom!
I've been having a similar problem since the last update but it was also a big Windows update time for this computer too. I'm using GameEx Arcade and everything else is totally normal and working a-ok as usual except for the Mame .mp4 videos. I don't think it's a GameEx thing though, I can't get them to work in Windows Media Player either, just like above I get audio but a black screen, same as inside GameEx Arcade. All the other console's videos are working normally, it's just the Mame .mp4 that are black with audio.
Uninstalled and updated the LAV codecs, tried the Shark ones, tried the Windows Media Pack ones, can't get the .mp4 videos to work in Media Player. They play fine in VLC. And they played fine in GameEx for the last year too. In December there was a GameEx update that did a similar thing, black videos with audio for the Mame snaps, but then a few days later the subsequent update fixed it.
But then yesterday I did a big many months overdue Windows update along with the GameEx Arcade update and probably hosed up other stuff on this ol' Dell..
I feel like it's a codec thing though.. any other tips to try?
PS - Thanks very much to Draco1962 for the awesome Arcade theme, it's perfect, couldn't ask for a better background for my arcade!