I am using Immersive reader in Instructure's Canvas LMS and not in my own application. The document https://video2.skills-academy.com/en-us/azure/applied-ai-services/immersive-reader/how-to-configure-read-aloud#configure-the-voice describes only changing the configuration (similar to changing it via the GUI) and does not indicate how to change it from within the HTML content itself.
For MathML see the example at https://canvas.kth.se/courses/11/pages/mathematical-expressions-can-be-handled