I have learned indirectly that some students find that I am soft-spoken in class, so they have trouble hearing me.
A long time ago, I was a really loud teacher. Then I took a few voice lessons, became protective of my vocal chords, and dropped a good number of decibels. Not enough voice practice to learn to project my voice far while keeping it nice and relaxed, unfortunately.
I also like to believe that a relaxed voice sets the tone for a more relaxed classroom atmosphere, where people are less nervous about asking questions. Whether that's true or not, I have no idea. (Has it been studied, even?)
So, what to do about this?
I suppose that a microphone could be the answer, although I am not sure what effect it has on class dynamics. It might become more like a show and less like a group endeavour.