I believe the “Couldn’t find file” errors are caused by using the top level ‘FMOD’ folder in the Additional Non-Asset Directories to Package/Copy.
On a side note, only use Package or Copy not both as it is essentially adding the files to the built game twice.
The reason we use the top level folder, is because we cannot specify different paths for each platform from a plugin. This can be achieved by modifying/creating config files for each platform in your project.
After looking into this further, it is definitely something we could document a bit better.
I have made a task to add this to the docs.
This is what you should see in the DefaultGame.ini (among other things):
Using these config files you can specify the exact bank folder to use for each platform.
eg. FMOD/PS4, FMOD/Mobile, etc.