LX4 is the latest, original factory ROM revision
The 'X' refers to Export.
Originally, WMS released games with ROMs for multiple markets. LA4 would be American, LG4 would be German, etc.
When the internet became popular, and people started to want to download the ROMs, WMS made the eXport ROMS available for download, as opposed to each and every language. The X ROMs have all coinages, etc in them.
Originally, DM had sound ROMs in U2-U7. U8 and U9 were EMPTY. When the DM 'home' ROMs were leaked (the ones with swearing), the used U8 and U9 as well, and supposedly the audio quality is better as it uses less compression than the original factory ROMs.
I thought the custom ROMs used the factory sound ROMs, but I could be wrong. I didn't think there were special 'demolition time' sound ROMs.