How do I keep audio track ONLY and delete video

One option is to go into clip inspector, just click on the little blue box at the beginning of your project/clip then choose Video.
Slide all the video settings (exposure, brightness, contrast, saturation) to zero which will blank out you picture.
Z.

there has to be a better way, but try this.
Make your movie, share it to iTunes.
then open up garage Band and make a new project.
open up the Media Browser in the bottom right hand if the window, go to Movies -> iMovie sharing -> iTunes and find your Movie, then drag it into the Imovie window, and it wil import both the audio nad video track ( video track as thimbnails). you can then delete the video track, then you can export the audio track to iTunes. I tried this and it seemed to work.

roger

There is a simple way. Create an iMovie project. Make your project as normal by dragging the clips you want from the Event.

Then SHARE/EXPORT USING QUICKTIME. In the dialog box, choose Export: Sound to AIFF.

This will give you a relatively uncompressed sound file with no video that you can further edit in GarageBand to create an MP3 or AAC. You could also drag the AIFF file into iTunes and convert it to MP3 or AAC.