Revit generates a "walk through", that is a just an AVI video file it cannot attach the audio to the video with in Revits native programming. You would want to open the AVI file after exporting it from Revit in a Video editing software that has the ability to add audio tracks. Then save it as a new video file format based on the software you used to edit the video with. You would not be able to reinsert that video back to Revit. That file type is only one way and its export only.
