Ok, pretty sure I know what is going on here. Also guessing why you haven’t responded to the above, because I’m reasonably sure that the problem you described solved itself.
First, couple of things from my opinion. First is that the sound that syrinscape is amazing and I don’t know of anywhere else that gives the same effect.
But then, the relevant portion - the servers are simply horrible, and frequently do not respond when queried. Either to play the requested sound, edit the sound so you can customize it, or to download a list of sound (relevant here). Which is highly likely the issue here. FGU when you attempt to import the information from the syrinscape servers to add sound links either to the story or book panes, sends an information request and waits for a response. If FGU does not recieve a response within a certain amount of time - either because the syrinscape server never processed the request, or did and began the download but froze mid way (also common), it will time out. At that point, you’ll get a message like the above.
Under conditions where “glitching” is common, it is far better to manually download the XML from the sound control panel from your web browser, and import that to FGU. Reason is if no data starts transferring, you’ll see that on your browser and can know that syrinscape has failed to transfer information again. Same if the download freezes, you’ll know that.
Long story short, especially depending on the time you’re attempting to import to FGU, it can take multiple attempts to get this to work. Off hours work better.
The good news (of sorts) is that for some reason, once you get the sound in and put links to activate it from the book/story pane, it will frequently play much more reliably than if you attempted to activate it from the web brower interface. Frequently plays with less delay, and more reliably plays sound.
Syrinscape is very much a jekyl and hyde service. Sound is awesome. Trying to play, import, and edit can literally drive you nuts though because of poor server response.
Also, for anything FGU related (which this sort is not, but still) I can recommend the fantasy grounds academy discord, many knowledgeable people there that can answer most FGU questions.