My "natural" voice is not monotone, but is fast, high pitched, and has an unusual cadence (can relate to the other person who described theirs similarly). But I learned very early on in childhood, that my way of speaking was unacceptable and weird, so I copied other people and for most of my life I have learned to speak like the person I am speaking to (not to the point of obvious mimicry, but enough so that if you heard me speak to different people at different times, you can tell I am doing it).
This creates some interesting moments when speaking to more than one person at a time (having to choose who to speak like), but I find people like you more when you sound like them. It has gotten me out of a lot of jams.
I also relate to the people who said they have nice radio voices, as this was one of the side effects of my lifelong imitation; I developed a lovely broadcasting voice & have been doing radio voiceovers for over 12 years. Make your weakness your strength, and all that. But yeah, normally I speak like a hyper five year old in private.
Biggest takeaway is that whatever you speak like naturally, you may be able to change your public speech patterns/tonalities if they cause problems. Doesn't mean you don't have Asperger's or that it's not difficult.