Last time we saw how to install PicoTTS to make our Raspberry Pi speak. Using PicoTTS (or any other Text To Speech engine you like) and the Yowsup class to send media files that we already used for pictures, we will be able to send vocal messages from the RPi, instead of just text answers.
In the future, if Whatsapp will really enable audio calls (and Yowsup will support them), it would be possible to integrate also a vocal recognition module, so we could ask something to the RPi directly, with just a call and hear the answer.
What a wonderful task from such a small device!