How do I create an export plug-in that ouputs the audio of a video as a wave file?

You can automate this process by using Adobe Media Encoder's Watch Folder feature. This allows you to set up a folder where any video file added will automatically be exported with the desired settings, such as audio output in WAV format.

For detailed instructions, refer to this guide: Create a Watch Folder in Media Encoder.

Do you want to export a rendered file? 

If so, Premiere does that. 

Do you want to make a file in realtime from the Premiere output as it plays?

In that case you need to make a Transmitter plug-in. 

As Bruce wrote, it´s already there.

I need to develop a plug-in that ouputs the audio of a video as a wave file. Do I need to assemble the wave file "by hand", or is there a suite that builds wave files?

By @r8dBlAckTreE