Notice what happens when you change that unenforceable command into an enforceable statement. "I'll be happy to listen as soon as your voice is calm like mine". Now if the child continues with the same tone, you can let them know that it looks like you'll need to try again later. With some repetition they get the hang of things pretty quickly and realize that staying tuned in to what you say you're willing to do, provide or allow is a valuable source of information for them.
On days when things aren't going so well we might try to set the exact same limit, by saying with exasperation, "You use a civil tone or don't bother talking to me, because I won't be listening." Even though the limit is the same, because it is framed negatively (and usually has a petulant tone to it as well) it pulls the kid into a power struggle with you, if you weren't already in one to begin with. Love and Logic uses the mnemonic Thinking Words vs Fighting Words, which I think captures pretty nicely what we're shooting for.
On Love and Logic.com they have a nice Enforceable Statements PDF handout. Be patient with yourself as you try these. Awareness is the first step. If you're trying out enforceable statements and you end up saying something much less helpful, remind yourself that this is the normal process.
Just being aware that there is an alternative, and seeing in retrospect where it might fit is progress that will start you on your way, even if your behavior has not yet changed. Give enforceable statements a try. They can be one important piece of a better relationship with your kids.