I had the same problem. I would like to take a moment and thank immo & softball10. You both are awesome and a Project-Saver
I had a video project that I had to revisit, and do a revision for, that I haven't looked at in about 2yrs. lol. Well, I was greeted by the yellow triangle on all of the audio files. I used a lot of audio samples throughout the 20 min piece. The idea that I would have to re-sync (retime, recut, etc) all of my audio was not a happy thought. Luckily, I searched for this problem. Because of immo and softball10 I was able to fix the problem with minimal effort.
Since the last time that I worked on this project I realized that I had done some Hard drive cleaning, moved and consolidated my music library. I had moved my iTunes library to an external HDD, and some straggler files were consolidated into my, now relocated, library.
I followed immo's suggestion to go through the Exporting process and iMovie was not only smart enough to warn me that Audio was missing---and the path it was missing from---but it actually was able to find the audio files on my external HDD, and all I had to do was confirm that the file was the same.
Problem Almost solved.
I noticed a warning that two audio files could not be found, and it showed me the path that they were missing from. I knew that these files were some straggler files that were now in my relocated iTunes library. So I followed Softball10's instructions and recreated the missing directory. I started by taking a snapshot of the screen---So I had the path down exactly. Then I recreated the missing directory iMove was expecting, and copied the audio files to them. I restarted iMovie and…No problems. ALL audio played! 😀