720 Pixel aspect problems

I shoot everything 720/24PN. After shooting I sync up the boom audio on the FCP timeline which was recorded separate from the camera. Then I export the video and audio to create a new clip with only boom sound to replace the old one using the 'current settings' selection.

When I import the new clip and bring it back to the same timeline for editing, it requires a render before playing back. Why? I can't figure this out. For some reason the pixel aspect changed from HD960X720 for the raw footage to square pixels for the new clip. Also the frame size changed from 960x720 to 1280x720.

Does anyone know what I'm doing wrong when I export?
 
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