When users lose their voice memos in their iPhone 6, they can have a look at their iTunes first, maybe they have 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 from iPhone 6. 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 6. The voice memos should have been restored to your iPhone 6 as well. However, as is known, there is chance that the iPhone 6 users will lose the newly added data in their device if they haven't updated their backup. At this moment, people will need a piece of iPhone data recovery software to recover deleted voice memos from iPhone 6 backup.
For more methods, check: How Can I Recover Deleted Voice Memos from iPhone