Ok, we had the same problem today, and the other answer here isn't really the solution...
This exception happens because the SoundBanks path(s) inside Wwise is set to the StreamingAssets folder inside Unity, which is where they get copied on build.
So I discovered two things which are not very clear from the docs:
- SoundBanks path(s) inside Wwise can be outside the Unity project (even better because this way the Unity Editor doesn't have to track unnecessary files), and all the sounds still work in the Editor.
- SoundBanks path(s) inside Wwise should be outside of Unity's StreamingAssets folder if you have checked the option in Unity Project Settings > Wwise Editor > Asset Management > Copy SoundBanks at pre-Build step, because this way Wwise copies the SBs from the generation folder to the StreamingAssets, and it only copies the necessary platform, which is being built, so in this case the two folders are the same so the exception is thrown.