The files at issue are not the .psd files; they are the asset/timeline or other files.
mp3 files may have metadata or other info that can cause problems. Some may work; some not. But they are not a good idea for menu audio.
Can you confirm that the mp3 names were short and without special characters? I want to be more confident that it was the mp3 files themselves. I’ll try to test whether some mp3’s work and other don’t.
Edit: Also, I missed that this was a flash build. Did you get the same error for a DVD/BD build if attempted?
Stan Jones