Hello!
I had the same issue with the new event based system. Found a solution, but i am not sure what the "official way" is how to do it.
The problem is the AkInitBank is not getting packaged in the cooking process. You can confirm this by unpacking your .pak files from the \Content\ Folder of your cooked build.
Please see
https://forums.unrealengine.com/community/general-discussion/105459-unpacking-a-pak-file on how to do it.
After unpacking, go to the \Content\ WWise folder where your Init Bank Asset is supposed to be. You won't find it.
To Fix it, you have to go to \WWise\Source\AkAudio\Classes\AkAudioBank.h and go to the Class UAkAudioBank.
Replace the UCLASS() with UCLASS(meta=(BlueprintSpawnableComponent))
Compile the project.
Now make a UAkAudioBank hard reference to your InitBank in some file (e.g. GameInstance or GameMode) which also needs to referenced / used somewhere.
Right Click on Your Init Bank Asset -> View References, in the new window, it should show a reference left of your asset.
The Init Bank is now referenced and should be included in the cooking process.
Would be nice to know what the WWise Team thought what the "official way" is, couldn't find anything in the documentation.
Cheers
edit: Also i've renamed the Bank to "Init" only, because i've noticed they're scanning for a hardcoded "Init" name Bank
constexpr auto AkInitBankName = TEXT("Init"); in AKAudioDevice.h
But i think the actual fix was the hardref