@Simon Lightwood Thanks for the question. Based on your scenario, I believe you would like to use speech studio text to speech studio's audio content creation feature to key in text to create audio files from the studio that can be exported as audio files to embed in your video for narration?
Yes, you can do this. If you are using windows you can use the photos app and create a video project to add music or narration into the video you created. The data you use belongs to you and can be deleted anytime from studio or other service. All cognitive services are governed by the compliance and privacy settings as defined. I hope this helps.