Can't extract because no permission?

I'm glad that you were able to get the pointers needed and that you got it working. Most of this is covered in the sticky posts on the forum but the constant 'improvements' that Apple make in security cause the behaviour of sandboxing to change with each release.

It ended up being a media locations issue. Once I shared the volume with the FCP project, everything worked. It's odd that the first few files in the project were read without that, as they were in the same volume.
X2Pro really needs a basic owner's/operator's manual that clearly describes how to get started and how things work, especially the whole sandboxing/locations issue. If I hadn't received a tutorial from another filmmaker, I would not have figured this out.

When you say that the file is 'the same everything' as all of the others, does that include being in the same directory on the same disk as all of the others? The initial error says that it is a permissions problem which suggests otherwise. Have you tried adding a media location to cover the location of this file?

Should also note that other .mov files are read without issue - from the same camera, same time, same everything.

Am trying to export audio files from my 71 min film but keep bumping into this error with files:

"An error occurred converting Final Cut XML to AAF

Writing AAF file
Adding assets to AAF file
Adding essence for asset [_JDR9516] to AAF file
Opening audio extractor for portion 0 of asset [_JDR9516]
Failed to initialise audio extractor with default bit depth of 16
Error Setting up for audio extraction (The file “_JDR9516.mov” couldn’t be opened because you don’t have permission to view it.)

Suggestion: Does this movie contain an unsupported audio format ?

No AAF file has been generated.
X2ProLE Audio Convert 4.5.6"

This is not in a compound clip, has no filters/audio adjustments and I've even reimported the clip (and others), and even deleted the audio portion of the track with no effect.
Can't upload the xml as it's bigger than your max file size.
Help!