So could talking to yourself be a stim?
I speak things out loud when alone, in a normal volume voice, but usually I repeat the same sentence or paragraph over and over and over. It can be a thought, a memory or something I wish I had said. Things get stuck in my head, and it soothes me.
Its better if I'm looking in a mirror, but I usually don't have time for that because I'll stay there for hours. This has been a problem looking in the mirror before going to work sometimes....late again.
If I'm out, then I know I can't do it, but stopping myself takes such an effort that it can bring tears to my eyes or just makes me run for somewhere private (like the toilets) where I can whisper at least, although I know I risk the sound being heard there, which makes it less comforting.
Its not voices in my head or me talking to someone else, it is just "thinking aloud". The repetitive thoughts are there already, i don't know why I need to hear them aloud.
Often in the car, when I've just come out of a social situation like work, I cover my mouth and talk, or just sing. I tried using an old bluetooth earpiece so it would look normal, but I couldn't stand the feel of it in my ear, nor earphones
I'm getting brave enough now to pretend I have a "stand alone" phone kit in the car.
Its not listed anywhere as a stim, but I really can't help soothing myself this way, maybe its not listed because its a female stim? My mum is the same, tries to hide it, but often is heard from another room, then you walk in & she's sitting at a mirror.........