Amber Heard says her voice has felt drowned out since the Johnny Depp trial ... lamenting the loss of her story in a new documentary. The actress appears in the new doc "Silenced" which premiered at the Sundance Film Festival Saturday ... and she reflects on her legal battle with her ex-husband in the clip -- including the impact it's had on speaking her truth. Heard said, "I didn’t understand it could get so much worse for me as a woman, using my voice. I have lost my ability to speak. I am