Since Adolf Hitler was Austrian, he would've spoken German with an Austro-Bavarian accent. However, my German teacher once commented that Hitler tried his best to get rid of this accent and speak in a more "standard" way. Is this true? I don't speak much German and can't really distinguish accents when hearing Hitler's speeches.
I am a german, so I feel I can comment here: I do not know if he had an accent while speaking normally, I can only comment on the speeches I have heard.
I cannot hear a german or austrian accent in any of his speeches. He has a distinct way of speaking, but if that is due to a masked accent or due to him wanting to sound interesting to his listeners I cannot say.
I'm certainly not an expert in German accents either, but here is a link to one of the few known recordings of Hitler speaking conversationally. Someone who knows more about the dialect can listen to it and tell you more.
This is not true. In his last speech you can still hear the distinctly Southern rolled R he had back when he first came to power.