My understanding is that the built-in math editor is not accessible and is not compatible with screenreaders. A proposed solution is to enable MathJax.
MathJax is a display engine for mathematics that works in all browsers, is compatible with screenreaders, and provides zoom. Enabling MathJax would allow Canvas to automatically recognize mathematical commands/formulas and display them. Other (potentially) attractive aspects of MathJax include the ability to click/toggle through a sequence of text (e.g. hints/directions for exercise), add mouse over text (e.g. remind students of important definition), and shorthand for typesetting chemical equations.