Really no consistent methodology working in PDF-1 / PDF/UA-1.
Once part 2 of the ISO Standard for PDF then PDF/UA is published it'll be significantly more doable.
Hint: Do know your Math MML.
The reader is still reading the text, even though the text is included in the Figure tag. Technically, I don't think it should be doing this, but you have tagged text as a figure... many advise users to "flatten" figures (i.e., convert to bitmap). Perhaps in the Content Panel this formula text is still in nested in a P or other tag and somehow this is confusing the reader? I don't know.
Convert the text to a bitmap, replace the formula with the bitmap image, tag that as Figure and that would fix the problem.
Insert the Alternate Text as Actual Text may also fix.
Just a few thoughts
- accessibility is not just about blind people or screen reader users
- undesirable behavior of screen readers should be fixed in screen readers
- screen readers have many options anyway, it might be possible to configure in a screen reader tool whether and how a screen reader reads text inside a figure vis-à-vis the Alt entry
- Acrobat's read out loud feature is not a screen reader (it doesn't even get close, and has lots of issues anyway, such as being non-functional on Mac)
- turning text into a bitmap reduces overall accessibility (tends to decrease legibility, makes it impossible to select and copy text)
As CtDave pointed out, there is currently no 'official' way to represent math in tagged PDF (and no, it does not work perfectly well in HTML5 either).
I'd suggest any pragmatic approach that does not also reduce the quality of the content or its representation is a good way to go. One option may be to put the equation in text form (as it would be typically spoken) into an annotation associated with the piece of math. It could also be useful - depending on who the target audience is - to also put the math there as LaTeX and/or MathML (AFAICT from my few encounters, blind math students for example tend to deal with math expressions preferably on the basis of LaTeX, much easier to read and write than MathML)