I can identify with you. I spent my childhood as an only child in an adult world - single mother, university professor. Often farmed out to families she knew when she was at seminars, sabbaticals etc., sometimes for months at a time. I learned quickly that being quiet and non-expressive was my place, and I learned that lesson well.
There is nothing worse than fake sounds of passion. I tried too, and my Domme just laughed. It sounded so ridiculous, worse than fake porn stuff. I can see why trying to do that makes you cringe! Rightly so. I also understand that this is not how you want things to be. I'll give this my best shot.
I too was quite poker faced and inert in any BDSM sessions I had - which only began when I was in my late 60s. I can't say what will help you, but I can tell you what worked for me. In response to my partner's uncertainty and frustration, I started giving verbal feedback about what I was feeling. That felt awkward too, but at least it wasn't absurd. It was a way of owning what I felt, what I wanted, expressing it, asking, directing my Domme's actions for my desires. That was a huge step for me - I wasn't totally comfortable with even having sexual desires and needs, part of the "good boy" syndrome. She responded wonderfully.
I am a switch, although I lean toward the sub side. I know from my Dom experience that it is essential to know what the sub is feeling. After all, in a truly loving scenario, the Dom/Domme is the servant, taking the sub on a ride to a destination of the sub's choosing. Like a limo driver, the Dom is at the controls, but the trip is in service to the sub's desires. I wanted so much to give her a great experience. I was overly cautious, and she encouraged me to do more - harder strikes, more pressure on those nipple clamps, stretch that hole a little more, "Hurt me!" etc. That direction helped me so much, and soon, I began making an intuitive connection with her. I could read her, knew what she was feeling, because of the verbal feedback, and then because I knew her, her reactions, different "tells" that revealed what was working and what wasn't. Her responses helped me to be comfortable in the Dom role, and to sharpen my skills. I was so grateful for that.
That information gave me the courage to get over my awkwardness and begin verbally giving that kind of feedback. I tried to be as open about my responses to what she was doing and my desires as my body was open to anything she wanted to do to me - chained, spread eagle, standing, everything totally accessible and vulnerable. "That's good." "More" "Too much!" "Keep doing that." "Could you ...?" "Fuck yeah!" It felt so weird and awkward, but it worked. Not only did she love it, and to focus her attention where it did the most good. It also helped me to embrace what I was feeling, and reduced my level of shame immensely. I got more comfortable in my own skin. It was visible - I was relaxed.
Then, slowly, I began to respond without filters to the sensations that were coursing through me. My body would rise to her touch when she strimulated me, when I wanted more pleasure/pain. I would twist and pull on my bonds if I wanted to get away from a series of impacts. I wasn't thinking about anything - just feeling and reacting. My breath would catch at a moment of intensity or surprise. And the natural sounds began to happen. I wasn't choosing or trying to make them - feeling comfortable, and embracing what I felt gave rise to the natural, unfiltered expression of the moment.
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My advice, for what it's worth: Put those feelings into words. Get over the barrier of shame and embarrassment by owning and expressing your emotions, reactions desires and needs. Verbal feedback may seem awkward to you, but a loving partner will eat it up and be hungry for more. They want to give you what you need, and need to know how to do that. If you get over that hump, you will gradually become more comfortable with yourself as a kinky, weird, sexual creature. As that level of comfort grows, the inhibitions will begin to wither and lose their power. We are naturally expressive. Look at the exuberance of a kid who is secure and comfortable. That can be you.
One other possibility: ramp up the intensity of the sensations. At a certain point, it's hard to be quiet and stoic. Let the noises happen when they want to come out. But the more you are comfortable in your own skin, and accept the mix of emotions and sensations within yourself, the more expressive you will become.