Combine Voice Memos in iOS
The native Voice Memos.app doesn’t offer this. You will need to upload your recording to a computer and use a audio editing application like Audacity I’m not affiliated with the software.


  iTunes does not have a way to do that.  You can do it with an audio editor.  A free one that many people like, and which has Mac and Windows versions, is Audacity.



https://discussions.apple.com/thread/4707813