Simon Baron Cohen writes about this in Mindblindness, which you can order from Amazon. I'm sure it's also in some other books about autism. Basically, neurotypical people (I'm NT) have this ingrained need to attract the attention of other people. So as young children, we learn to do this "rollercoaster" thing with our voices that makes them easier to listen to. An example of this can be seen in "My Fair Lady" when Eliza is learning to say "How KIND of you to let me come." The assumed reason that people on the spectrum do not learn to modulate, or vary, their voices is because they do not have an ingrained need for others to pay attention to them. As young children, they simply don't care. What they say is more for their own benefit. When people with ASD get older and realize it's important for to communicate, it takes an effort of some sort (more for some people than others), so it's never going to be as easy as NT people, who don't have to try to do it. I think a lot of people never realize they have a "flat" voice and it's not a big deal.
Here is a lengthy video explaining how to modulate your voice. I like this because it explains what is going through the heads of NTs when they do it. So it might be helpful in understanding what your friends and family are expecting.
This one looks much more useful to me for an ASD audience. I think you could actually use this to relate to others, improve interactions, and work on simple modulations. It also includes simple body language tips, which can be confusing to many people.
My opinion, as someone who has many family, friends, and exes who are aspies- is that you shouldn't worry about this too much. If you're really worried about it, you could record yourself and see if you think they're right. If it bothers you, you can work on it. I guarantee you that people value you just the way you are.