Be the noise. Note different sounds in your neighborhood, then act them out together. The more your toddler can understand what loud and quiet mean, the better he'll be able to replicate them himself.
Tell a secret. If you whisper an invitation to sit down for a snack, your toddler may quiet down to hear you -- and may want to whisper his requests for more cookies.
Pump up the volume. The next time you head to the park, proclaim, "I'm turning on my outdoor voice now!" It'll show him that it's okay to be nice and loud...sometimes.