This feature is only available for Synthesia Creator and Enterprise Plans Customers.
If you would like discuss upgrading your plan, contact the Synthesia Support team or see the How do I upgrade my Synthesia plan ? guide.
To download the audio from your video:
Navigate to your generated video and click the 3-dot menu, located at the top right of the page.
Select the Download Audio option. The audio is downloaded as a wav. file into your Downloads folder.