I think the "monotone" voice of AS is a poor concept.
Yes, I tend to talk in a monotone voice, but does my voice get emotional? Yes. Very easily if it's one of those things that set me off.
Typical AS behavior is either on or off. If something makes you emotional, your voice will reflect that. If it doesn't, you remain monotone.
"Normal" people tend to have more dynamic speech because their emotions flow in more varied amounts.