Shelly,

My DS8 talks, sings or taps things to the extent that at times it feels constantly to me. He was always intense and physically likes constant motion but the weird silly voices started around 5 or 6 and are still there. Like you said he does not use this voice in conversation with me except rarely. Unfortunately he taps and talks and sings all through the school day as well annoying his classmates. He is also physically "rough". I don't mean aggressive or hostile just likes to bump and thump and hug hard etc. Add this to being several years ahead of them in most subjects and correcting them you can see that he didnt' win a ton of friends. On the other hand there were days when he had the whole class and teacher laughing with his antics. We are looking now at possible causes (ADHD, Sensory etc.) and also just about to start social skills/ emotional counseling for him to learn some tools/rules and to have an outlet for any emotional issues.
He was diagnoses with Sensory Integration Disorder at age 3 and is considered a "sensory seeker" which might explain the physical stuff and perhaps the voices, who knows? I like you worry about the effects of all this on a kid already "different".
And as a final note, as I type this he is in the other room talking like SpongeBob on Speed. Sigh.........
Let us know how you make out. I sincerely empathize.