We recommend generating the SoundBanks outside of Unity's assets folder. The reason for that is that there is no way to specify platform-specific files in the StreamingAssets folder. Everything in that folder gets copied in your built game. If you have the SoundBanks for multiple platforms there, they will end up is all your builds, resulting in bloated game size.
When iterating in the Editor, the integration will actually load the banks from where Wwise generates them. In your case, the integration looks in "WwiseProjectName/GeneratedSoundBanks", and loads the banks from there. In the build game, though, the banks have to be in the StreamingAssets folder. We recommend manually copying only the SoundBanks for the platform you are building within StreamingAssets, build the game, and then delete them from StreamingAssets.
There is a way to automate this, though, and we provide an example script do accomplish that (UNITY_PROJECT_ROOT\Assets\Wwise\Editor\WwiseMenu\Common\AkExampleAppBuilderBase.cs).
For more information on all of this, you can refer to the Unity integration documentation (Help > Wwise Help), under section Wwise Unity Integration > Build for target platform.