After some testing, it seems that what you’re running into is either an issue with how the FMOD plugin saves the newly generated assets, or a limitation of UE’s built-in source control integration. Either way, svn:ignore properties set externally aren’t respected, and generated folders and .uassets are always marked add when Source Control → Submit to Source Control… is selected.
I unfortunately can’t recommend any workarounds besides manually undoing the add using your external SVN tool of choice after the assets are generated in UE and marked for add, or simply not using UE’s source control at all. For now, I’ve passed this issue to the development team to see whether there’s anything that can be done about it.