How can I get back voice memos that I deleted from my iPhone 6?
Besides using iPhone backups via iTunes (explained more here and here) there are applications you can download that will allow you to navigate your iPhone and files that you are not allowed to normally access. iFunBox is one that comes highly recommended by many.
However, with the iOS 8.3 update, many file managers cannot access applications like they did previously. iFunBox released an update that allows users to explore app data again, but only those that have iTunes File Sharing.
I would suggest trying a file manager application first to see if you can retrieve the voice memos before you implement an older backup of your iPhone, since the backup will overwrite your phone and if you can't access the Voice Memos through the backup, they will most likely be lost for good.
If you did a full backup, then yes, you can recover your voice memos again. I found this even works across phones when I have tried it -- awesome! But if not, they are lost for good, afaik, and you cannot recover them again. They're gone.
This is why I usually consider Voice Memos obsolete, and would recommend recording voice snippets with the Camera instead (cover the lens if you want) and leave Voice Memos alone. The reason is that media is generally backed up to iCloud more often, and therefore is much harder to lose, while making it easily accessible on your Mac, plus other devices, and cloud account, obeying the "three places" rule. Of course, for better privacy, you could simply carry a real voice recorder, or another phone/PDA/tablet/other mobile with a SD card slot and record on that.