WRITE IN YOUR OWN VOICE
Be authentic. Have an authentic voice. Of course, having an authentic voice is an aspect of your sense of your authentic being, of knowing who you truly are, of being truthful with yourself and truthful about your writing. When your writing is authentic, you are grounded in your truth. When you are grounded in your truth, in whatever aspect of your experience, you can speak in your own voice, and when you can speak in your own voice, you can write in your own voice. Writing in your own voice is a measure of your authenticity.
However, a skilled storyteller can adopt many voices, because the skilled storyteller will utilize such writing conventions as narrative point(s) of view and narrative voice. With regard to narrative voice, the skilled storyteller can assume the voice of any other persona or personality. But even the assumed voice of that narrative personality needs to emerge from the authentic experience of that storyteller.
This is complex skill the true storyteller develops over time.