There is a very easy way to improve your voice: get used to listening to it. You must be aware of the sound of your own voice. So, every time you have to prepare a talk or an oral exam, record your voice and listen to it. Or choose some literary texts, like a poems or a piece of prose, to read aloud and record. Speak or read in a loud voice as if you were in front of an audience in a large space. Only then will you have a chance to understand what isn’t quite right and make any necessary changes.