Modify this node.js code to output .wav instead of .mp4?

I'm trying to modify the following code to output .wav instead of .mp4:

node-tts text to speech synthsizer, (OS X)

The issue is, I'm not a node.js developer and have no idea how to make the change, and if it is even possible. I'm attempting to speechify an app so that visually impaired users can use it if they are running the TTS server on their LAN. I need .wav output for my project.

Can anyone tell me if this is possible and if so, point out where in the code the audio file is actually generated? Presumably this code uses the Mac's built in speech synthesizer to create an audio file and this code just delivers it the the calling client.