It just takes practice. Hearing it out of context, it's just a jumble of words, but you can make out some things if you notice, plus if you're controlling it you know what to anticipate, so it gets better with practice.
Exactly. My screen reader is very adjustable in what it reads. When you have that level of control, you will be able to anticipate some of what you will be hearing, effectively improving the speed at which you understand. It's just how you read with your eyes (i.e. an experiment has found that when reading headlines, people read the first three words and last three words first).