Articulate Presenter stores narration in the *.ppta file. You can export a copy of it using the built-in audio editor. Here's how.
- Open your presentation in PowerPoint.
- Go to Articulate > Audio Editor.
- Click the round Articulate button in the upper left corner, scroll to Export, and choose WAV or MP3.
- Browse to a location, and click Select Folder.
A separate audio file will be created for each slide in the presentation that has narration. Here's a brief video screencast demonstrating this process: