Remove the media card, and re-insert it into the BB, and see if the files show back up. If they don't, it's possible that the file system on the card has been corrupted somehow, and I doubt you'll get the voice memos back
I doubt Disk Utilities would be able to sort it out. A Windows machine might (but I doubt it, too). It's worth a shot if the files are important and you have ready access to a Windows box. I would suggest copying all the files off the card (using Mass Storage Mode), and then (re)formatting the card in the BB, and copying the files back into the appropriate folder (DO NOT just copy the folders back onto the card and overwrite the folders on the card). Won't get your memos back, but it should fix any file corruption problems on the card.