So I don't know about the MacBook but I was able to recover the recording from the iPad using DiskAid: http://www.digidna.net/products/diskaid It essentially lets you use the iPad (and iPod touch) as an external hard drive. I found it in the Evernote folder. In the future, it would be great if the file wouldn't just disappear if it can't sync, that caused some major stress.
I made an audio recording in Evernote on my iPad, and when I stopped the recording, the file vanished. It's not in the trash, or in the note. Is there a temporary file folder, or any way to recover the recording?