Current questions and answers regarding the map editor.

First, you need to record the voiceover and save it in wav format, placing it in the Complete folder (if it doesn't exist, you need to create it). Next, create a video, create a new sentence, and next to the "text" field, there's a "sound" parameter. Click "New" and create it, then select the desired recording from the Complete folder.
Click "OK".
Without closing the sound editing window, find the sound you created in the left tree, right-click, and select "Export". At the bottom of the console, make sure there are no errors. Now, click "OK" everywhere, close the window, and it should work.
(Compiled from AlekseyS's message and my addition)