Its not - its just that you can't subsequently Export them (the SoundLinks on a Map) to a Module. That's why I discourage people from using SoundLinks directly and instead recommend using them via MiniBoards.
While Soundlinks look and act like regular FG-Links, they aren't, and they don't Export directly, only indirectly via the MiniBoards. That's because the code to get them to work is over-written by the FG-Engine when they are re-imported. Remember that the FG-Engine is designed to work with Exported/Imported FG-Links, not the modified/special code that makes SoundLinks (and, incidentally, DOE: OLELinks) work.
BUT, by indirectly Exporting/Importing them via MiniBoards then the correct coding is preserved inside the MiniBoard and thus the SoundLink remain correct and working.
That's what I get for hacking a system and getting it to do things it was never designed to do - ie the FG Engine and regular FG-Links :)
So, use the SoundLinks directly on Maps if you want, but you won't be able to Export them and therefore I won't support them if you try :)

