same problem here. Brand new MicroFreak (Vocoder Edition)... can hear any preset fine, except vocoder related preserts.
Tried with the included Mic, tried with a TRRS headset too.
Edit with fix: I further checked the updated manual, under the Vocoder section 19.2.2.3 to be precise (page 119) and it mentions that you need to set gain first.
To do so, it's fairly simple: With the type wheel, set to Vocoder. Then press the Utility button, Mic settings, Mic Gain, and in my case, I've set it to Auto Gain (all the way towards the right).
Now I've got sound using any of the vocoder presets.