First get the notion to play. It's a little boring here at the table after dinner. Use your loud voice over and over until Mommy says "Liam, that's a loud voice! Let's try quiet voice now please."
Use your quiet voice -- this involves just moving the mouth and sort of blowing air out of it; almost no actual noise. When Mommy doesn't notice right away (it's easy to miss) use your LOUD voice again and laugh hysterically when Mommy says "Loud voice!"
Become beside yourself happy when Daddy decides to play too.
Look with wonder at Daddy's quiet voice.
Laugh hysterically when Daddy says "loud voice!"
Important game playing note: only Mommy and Daddy say the words "loud voice" and "quiet voice" and quiet voice is used most often because "loud voice" spoken loudly can be a little scary sometimes. Liam uses "de de!" (which also is what the birds say) for loud voice/quiet voice.