When you right-click a speaker icon (audio track) in Storyline, there's an option to Export Sound. This allows you to save a copy of the audio track as a WAV or MP3 file (or both).
Although the Export Sound option is available when you select multiple speaker icons at the same time, it'll only export the audio track that you actually right-click.
There is not currently a way to export multiple audio tracks at the same time from Storyline. If you'd like to suggest this feature, please submit a feature request to our product development team.