I created a custom sound for my leaf trap in the Autumn tileset. I inserted it into the Old Formats graphic set tool, because I'm more familiar with that one. The conversion of the tileset itself went fine, but the trap is silent in New Formats.
I know there is a sounds folder, and I put the sound in there, too. But I have no idea on how I have to name it in order for it to be recognised. This doesn't seem to be part of the translation table or anything along those lines.
If it's too difficult to make the Graphic Set Conversion tool take care of custom sounds as well, please at least make it part of the graphic set conversion instructions (https://www.lemmingsforums.net/index.php?topic=3393.msg65778#msg65778), so that any graphic set creator knows how to do this manually.
Look at the .nxmo file of the object.
After the "TRIGGER" lines add "SOUND *insert_soundfilename_here*" as a new line. The custom sound should now play.
If you need an example, look at the L2 space teleporter's ".nxmo" file. The line there is:
"SOUND teleporter"
But yes this should be mentioned in the topic as well.
The conversion tool does not automatically handle custom sounds; they must be restored manually.