The function PostEvent( ... ) will return AK_INVALID_PLAYING_ID in case of an error. When you are receiving AK_INVALID_PLAYING_ID, it means that the post event request was not successful, and it was for one of the following two reasons:
(99% of the time) The specified Event was unknown (either the event ID or the string specified was wrong(or a typo) ) or the bank containing the event was not previously loaded.
(1% of the time) There is insufficient memory to add the request in the message queue. (in such situations, a messages has been sent to the Wwise profiler to display that memory pool named "XXX" ran out of memory.
Now, it is possible that PostEvent returns a valid playing ID, but it is later, in the audiothread, that it will check if the media is available. It can only be detected at this time since the media to be played may be decided only later by an RTPC or by a random container.
The best way to identify the reason for the problems is to connect the Wwise profiler to your game and look at the capture log (you can filter things to have only "Error" and "Message" messages type to be displayed. You will receive more information in this log to help you identify possible problems.
