I found that in one situation this was because the event was being called before the wwise sound engine was fully loaded. Have you made sure to print all wwise messages to the console (including load messages?) Also, was the Audio working and then suddenly stopped? I also found this error after a bad crash in Unity and it corrupted something in either Wwise or Unity. I was able to resolve the error by restoring both programs and project files to a last known working version.