i tried to learn all of them for my trials and it didn't work. Pick 3 or 4 that are from different time periods (they like to ask you to compare speeches from different contexts) and different genders, and also different positions (eg. don't pick all politicians). That should give you a good range. I've found that Socrates, Goldman, and Havel work well together. Then just learn the general stuff about the other speeches - make sure you know a few techniques for each, the context of each, and the main ideas in each. If they do specify a speech they want you to talk about (eg. one year they specified Lincoln) it will only ever be a short speech such as Lincoln or Levertov.
I learn things well visually, so I did a mind map for each speech, with more info on the maps for the 3 i learnt in detail. Try and find a way that works for you, whether you need to do mind maps or maybe record yourself discussing techiques about all the speeches and then listening to it before you go to bed.
Just make sure you know something about all the speeches, because they haven't specified a speech in a while so this might be the year they do it!
Also, remember the speeches module is one of the hardest, so they will take that into account, so it can end up scaling you up more than the other modules might.