To reverse audio in voice memos, you can use a third-party audio editing application on your computer. One popular option is to connect your iPhone or iPad to your Mac, then use a program like iExplorer to locate the original voice memo recording file. The file will be in the m4a format and you can listen to it to confirm it's the correct recording.
Once you have the file, you can open it in an audio editing program like GarageBand or Audacity on your Mac. These tools typically have a reverse or invert function that will play the audio backwards. After reversing the audio, you can then save the edited file and transfer it back to your iOS device. This allows you to undo any accidental audio edits or replacements made directly within the Voice Memos app on your iPhone or iPad.