Hi,

if we are talking about built-in WebChatbot - it doesn't take the voice input, it allows only text. We have the component to integrate with Alexa (available on Exchange) - maybe it is something you need. 
Of course you can implement voice recognition in your app layer and pass text to the chatbot to achieve this. OOTB Pega WebChatbot doesn't take the voice input.

Piotr