It looks like slightly dodgy ring modulation.
I really hope that you understand each other for this “bug” 
It looks like when the signal exceeds the digital maximum or minimum it inverts the signal on the output. So rather than the smooth audio samples you see at the start of the second waveform diagram, which is how audio will always appear you get a very abrupt change in level from maximum value to minimum in one sample of the audio which is best described as a ‘splat’. The process of doing vocoding involves a lot of digital multiplications and it looks like the multiplication function is wrapping round the numbers wrongly as it tries to multiply numbers that are too big for the system…
Analogue kit overdrives nicely, digital kit is far less forgiving.
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The .zss file loads ok on a Pi4 but hangs on a Pi3…
And for those that haven’t done that before, is there an overall style of approach?
Understood.
But then, why there is not a “max value” to handle that ?
It sits at the edge of some else’s vision of the world and as such we try to develop these interfaces whenever we can. By reporting something like this the understanding between communities that may not even realise they share some context.
Look upon on it as code based jam session.