Have you upgraded to iOS 7 recently and tried to use the Voice Memos App ?
If you use the voice memo for long recordings (above 15 minutes) you may have noticed that when you finish your recording and it asks you to name it that when it has been named that th length of the recording shows at 00.00, and does not play back what you just recorded.
after much research and reading online including on the apple forums not one apple rep had mentioned when or how to fix this issue. although i do not have an iPhone anymore i still have an iPad and Apple TV, not that i use the memos but even so this still needed to be investigated.
So as a temporary fix (maybe permanent) if you have any previous recordings from your older iOS from before the upgrade, then make sure that these are backed up in iTunes.
Once they have been backed up then delete them all out of your voice memos app, and try a new recording i tried a long recording of about 35-40 minutes just to ensure that it was not a fluke that it worked, once it had finished recording and i had renamed it i then checked to see if it worked and it did.
I then proceeded to make another recording to see if the initial fix would work (the fact that old recordings were present) once again i made a 30 minute recording and then i renamed and saved and once again the recording saved and replayed fine, thus leading me to the conclusion that any older recordings made prior to iOS 7 are not compatible with the new revamped Voice Memos app for iOS 7, so until apple fix this issue i would recommend that you do the above to capture your meetings/lectures.