As actors there are two ways we can form a character. The first is with your voice and the second is with your body. Your own voice and vocal patterns are as distinct as a fingerprint. The pitch, intonation, accent, cadence, resonance and inflections are all products of you. They come from your self image, your upbringing, the length of your vocal chords (though of course, they can be trained), how you shape your breath and how you project your voice. Its part of your instrument and you can “tune” it according to how you’d like to be perceived both for yourself and for the characters that you create. Try using a tape recorder when you practice a monologue so you can hear how you sound. Only when you are aware of yourself can you begin to change it.
Here is the first is wonderful series of 12 videos by the National Theatre in London for breath, vocal warm ups and vocal techniques for actors. Enjoy!