[cdawzrd]

I haven't worked with Inventor too much but this is how I would do it:

-figure out how to export your presentation as a video

-open it up in premiere / windows movie maker / final cut / your video editor of choice

-record and add a commentary track (or record with your sound editor of choice such as Sound Recorder for windows, then import it into the video editor)

I don't know if you can add commentary _within_ Inventor..