This is a basic presentation principle. Vary the pitch. Vary the speed of delivery, slowing down for emphasis. Entice the students to want to add to the discussion by asking questions with a level of enthusiasm that is infectious without going overboard. Students will tune out if you appear too excited as well as if you sound bored. There's a middle ground that causes audiences to participate, to become part of the experience. That's what needs to be strived for. And even the most jaded student will find themselves caught up in the discussion if the instructor uses inflection well.

I use this method often as well as when emphasizing information that will be on a test.

You are right, by varing your voice volume, I have noticed this to be effective in getting attention.