I have echolalia as well. Hopefully it's in my mind most of the time, unless I'm in a high level of stress or alone. I count until 8 since I'm a kid when I'm stressed, that's the one which is the most common in me. 1-2-3-4-5-6-7-8, continuously.
I also mimic noises and repeat sentences/poems/movie dialogues.
I also repeat in my mind what someone just said (completely involuntarily, I don't even know it happens when it does, it's like a reflex) it helps me understand it and then find the answer, but hopefully it stays in my mind, people just see a lag between their sentence and my answer. That makes me look a bit weird I guess (some people believe I'm stupid easily because of this "lag"), but they don't know there's an echolalia in my mind. For me it's not necessarily connected to the sounds I will produce with my mouth, although the more stress there is the less I'm able to maintain this image.
I agree with your last point, it is involuntary, it just happens. It's not exactly something you're doing on purpose.