I personally find it very odd that there are unpublished alert types being pushed over the API. It seems that the API specification is much less strict than I thought.
Regardless, it is impossible to pre-record audio files for alerts that have not been published in the list of possible alert types, therefore SkywarnPlus does not support them.
This has never been an issue before, and I was not even aware it was a possibility for alerts to appear on the API that were not listed in list of alert types until you posted here.
Perhaps in the future, SkywarnPlus can attempt to quickly generate text-to-speech audio in the rare case that it encounters an alert that is not officially listed as a possible alert type (no pre-recorded audio file in library)