I am willing to share files with Apple. Mine are copyrighted, but are shareable.
But here's a wrinkle on the problem that puts me a little more in sympathy with Apple engineers, since they may well not have encountered the subtle environments we have that is generating this audio problem.
I was having the same problem you are for several days after I upgraded to Keynote 6.5 and Yosemite 10.10 (running on a mid-2011 MBA). I duplicated a keynote file with audio recorded that cut out after 50 seconds or so: audio stopped on slide 2 even though I had recorded and saved the whole 50 minute presentation. I store my keynote presentations along with Quicktime movies (m4v) and downsampled mp4 files in a sparsebundled disk image. I created a new, larger disk image yesterday because I was running out of storage space. To do that, I copied all the files in the sparsebundle to a temporary folder on a backup drive (My Mac Book Air is running out of storage space). Then, I deleted the old sparse bundle, created the new one, and copied all the files back.
After reading your latest post, I thought I would try again. I opened my test Keynote file, now copied to the new sparse bundle, and the audio works! I can hear the audio in the Keynote file when I "Play Recorded Slideshow" from the Play menu. I also exported to a Quicktime m4v movie and the audio is all there. The Keynote file size didn't change during all this: still 36.8MB, which is typical for the audio recorded ones. I start with a 2 to 4MB presentation of 15-20 slides, record audio, and the file size increases to 35-40MB. Then the Quicktime m4v is typical 200-300MB. This one is: 239.4MB.
So, although the bug is not squashed, take heart and don't delete those files! Your audio may actually be in there somewhere.