I've tried the Tal Vocoder, however there are two downsides.
1. It's not fully compatible with my screenreader. I can access the presets, but the buttons are bitmap-based.
2. The resultant sound is so low that I can barely hear it.
3. I use Cakewalk TTS 1 as the source sound to make the vocoder work.
4. Is there a free software vocoder without a GUI, that could possibly work with a screenreader like JAWS?