The voices are listed with ISO 639-1 language codes (
Wikipedia), followed by ISO 3166 (
Wikipedia) country codes for the local dialects.
You can go ahead and download any you'd like to try that start with "en", if you're not feeling like exploring other languages just yet. They've Canadian & Australian English in there as well, and there's a whole bunch more US English voices to choose from if you scroll down on the expanded list (ETA: oops, actually only 2 en-US & 5 English voices total, which from your post above, you probably already all have; scrolling artifact visual illusion, I guess).