Retrieving a file from an iOS backup
I would do the restore and get the memo (or sync them all to iTunes or another program like PhoneView)

That might take 10 seconds of your brain time to get going and some waiting. I've not found a speedy way to dig amongst the backup files to locate specific memos.

The backup files are stored plainly in ~/Library/Application Support/MobileSync/Backup/ but there might be 8,000 files with names friendly to the backup process and not to you and I.

Perhaps if you knew the size of the file, you could narrow down the audio file or drag a few hundred of the closest sized files into iTunes to see which of them are actual sound / movie files. Scripting the file command to determine which are sound files might narrow the search.