I heard a co-worker lower her voice

Context
The Haynesville Shale is a potential rich source of natural gas that's being explored in several parishes in Northern Louisiana. As such, it has been the topic of quite a bit of conversation and controversy. One of my co-workers was talking today about a protest she's seen some people engaging in that has something to do with the Shale. She lowered her voice when mentioning that the protesters involved were black. I said, "hmm," to all her comments.

Commentary
When I say, "hmm," during a conversation, it usually means, "I think I disagree with what you're saying, and I'm deciding whether I should tell you that or not." I mean, some things are not worth arguing about and sometimes I have a relationship with someone that I don't want to mess up with controversy. People who know me well have just read those words and been overcome at the idea that I might possibly avoid an argument, but that's because I argue freely with people I'm close to.

The "lowering your voice to say someone is 'black'" phenomenon is not entirely new to me, but I never get used to it. I used to tease a woman at my husband's former church because she would do it when using "black" as part of a person's physical description. I would say, "you know, it's not embarrassing to be black!" I wonder if she did it because the way we refer to race has gone through so many permutations during her lifetime that she could never be sure she was being mannerly.

In today's case, however, I'm pretty sure I know why my co-worker was lowering her voice. She was doing it because she was saying something derogatory about the protesters and she thought one of the black people we work with might walk into the lunchroom and be offended. What I can't figure out is why she didn't think I would be offended. That's what my "hmm" meant in this case. Something along the lines of "I'm not particularly interested in the topic or tone of this conversation. Next time you feel like you have to lower your voice in order to say something to me, please feel free to to turn it all the way down."

What did you hear today?