Some of the lowest quality voices, those synthesized by mbrola, have stopped working after an OS upgrade. These will remain nonfunctional because mbrola is a messy system and debugging it takes time. All other synthesizers are tested to work. In practice, you'll notice that ATR won't produce any sound if you pick one of such voices.
Not a big deal as ATR is gonna get orphaned by the new firefox web-extensions which break compatibility with older add-ons.