The simplest way to merge voice memos is with the Merge Voice Memos app.

This app works as an action extension so your recordings can be merged directly from the Apple Voice Memos app.

App usage instructions:
1. Open the Apple Voice Memos app and select the recordings you need to merge together.
2. Tap the share action, then scroll down and select "Merge Recordings".
3. Once the merged recording is ready, save the merged recording back into the Apple Voice Memos app or to any other destination of your liking!