I'm 27 and I live through a similar experience whenever I record my voice in message apps, when I record a voice message on my own without the thought on having to send to anyone in particular, I usually don't stutter, but when I do record it in someone's chat, that's when I suddenly can't speak and get my words straight. My interpretation of that is I fear their expectation and repercussion that I'm not gonna speak well. I think your example is similar, the expectation that someone else will have to hear and analyse your voice and performance is probably what's causing you to stutter