Using a custom vocabulary in Dragon NaturallySpeaking, I can dictate a plain text file containing this NatLaTeX source.
Basically, NatLaTeX defines a speakable form of many common LaTeX commands, including everything you need for most mathematical expressions. I use a system based on NatLaTeX to dictate all of my formal mathematics, including anything in that I'm going to turn in for my coursework. (Context: I have a chronic pain condition and typing is very painful.) The original question was about doing math without handwriting due to disability.
(This is an edited version of an answer I posted on Math Stack Exchange.
