Thanks. I'll upgrade after this project. I understand that PS 25 will also allow me to copy audio equalizer settings clip to clip, which 23 can't do.

For others:

I was able to work around this issue (of audio drop) by exporting the video in halves as Mp4s.

For some reason, after multiples tries, the audio exported complete in the shorter segments.

I then imported the MP4s into a single Studio Project, which did export complete.

Not much quality loss that I could see, but of course subsequent editing was made awkward.  

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