I just faced a version of this problem, and solved it, so I thought I'd share. I didn't see this solution in my searching for answers. I had dozens of memos and only wanted to keep a few.
I don't use icloud, but I do sync to my computer using an older iTunes, which works fine, so I can't speak for what newer versions of things do. Hopefully, some version of this still works for folks less Luddituously cautious.
- Set up iTunes to include voice memos in music sync settings.
- Sync, so all the memos are in the computer and on the phone.
- in iTunes, right click a voice memo and choose to open its location in your computer.
- You'll see all your memos there. Copy out whatever you want to keep.
- In iTunes, Go to Playlists, Voice Memos, Select All, and Remove from Library.
- On the phone, delete the Voice Memos app using Settings, General, Storage (I have seen indications that it does a more complete delete than the tap and hold method, but either way, it doesn't hurt.)
- Go to the App Store and reinstall Voice Memos. (Make sure you choose the real one with the correct icon!)
- You can now sync as needed and the memos should stay gone.