The problem is that I seem to have only two volume settings: One is quiet and calm, the other is loud (maybe medium-loud) and pissed off. I'm aware that I mumble, so when someone can't hear me, I speak loudly and clearly, with obvious enunciation, so I T-A-L-K....L-I-K-E....T-H-I-S, and hopefully don't have to repeat myself again. Of course, then it sounds like I'm being a smartass, which maybe I partially am, but the fact that I have to raise my voice in the first place is kind of taxing, as you pointed out/ noticed, so it does kind of get me annoyed.

A notoriously loud-spoken coworker once asked me "Why can't you speak up,
without the attitude?"