To proofread is to read for errors, you have to do it at your slowest reading speed. You should use any tools at your disposal. Text to speech may be one of those, but don't forget about traditional techniques like using a sheet of paper or an opaque ruler to focus on one line at a time. Some people also read the text backwards a a final pass, line by line.
I have my doubts about the efficacy of text to speech; it mispronounces many words and all the false positives get tiresome very quickly. But perhaps it would allow you to proofread while tired and distracted.
You shouldn't try to focus on everything at once during one read-through unless you have no other choice. You'll be less likely to detect inconsistencies.
Once you found and corrected an error, reread the line. There may be others lurking.
Read page by page. Take breaks.
And at least two pairs of eyes should go over the text.
Proofreading isn't editing (i.e., content and structure).
My two cents.
