It appears you may be able to until you click on Done in the Voice Memos app. You can navigate to Media/Recordings to see. That’s where you’ll find all voice memo files. Just open the folder for the memo (go by date and time) and open the Fragments subfolder. It might contain the overwritten one there, though if you had already tapped on Done, it appears those fragments get deleted, and just the final version of the memo is saved to the M4A file.
Then, overwritten is essentially the same as deleted once you click on Done. If you have already tapped on Done, your only other bet with iExplorer is if the memo that got recorded over was backed up in iTunes prior to it being overwritten.
As for only seeing data in backups from Dec 2014 and before, that would just mean that you haven’t backed up since then.