When you lose your voice memos in your iPhone, they can have a look at your iTunes first, maybe you've synced the voice memos to iTunes, and the voice memos can be found in iTunes folder. If not, you may need to restore your iPhone with iTunes to recover deleted voice memos. If you have updated your iPhone backup in iTunes, you will save the voice memos in iTunes backup as well, so you are able to get the lost voice memos back with iTunes.
When the restoring process is done, you will get the backup contents in your iPhone. The voice memos should have been restored to your iPhone as well. However, as is known, there is chance that the iPhone users will lose the newly added data in their device if they haven't updated their backup. At this moment, you will need a piece of iPhone data recovery software to recover deleted voice memos from iPhone backup.