I almost found the Holy Grail, Peter. (about dynamic sound control, of course). And, Glory to him!, is a cleaver JS trigger solution from Math Notermans. No issues with track alignment as there is only one track and the JS triggers handle the entire volume level. His file.story is here:
https://community.articulate.com/discussions/articulate-storyline/how-to-create-a-sound-volume-buttonThanks to him.
I recreated your demo with a very-very simplified copy of Math's solution.
https://360.articulate.com/review/content/10efc235-0853-49e2-9ccf-3b76b9e9281e/review)I only kept two buttons: high volume and clear volume. These are the same buttons for blurring and unblurring the background so you have both effects at the same time. Of course, JS triggers don't work in Review, so you need to download the .story file here:
https://www.course-design.fr/download Then click on the story.html file to see the demo in action or open the .story file and export for web. It works well. Let me know.
AND, I'm also answering Jonathan by adding buttons to manage captions without relying on the default player button (as captions are enabled or disabled by changing the built-in variable "Player.DisplayCaptions" (false or true ), it's easy to create buttons to change the variable as you need).