I can't hear you

Daniel has taken to speaking very, very softly. The lips are moving, but if sound is coming out, I can't tell it. So I have trained him to speak up.
When he mumbles or whispers, I reply, "I can't hear you, speak louder please." And he does.
I don't know what brought this on, or whether it's a phase he'll snap out of. But it was a quick trip from "use your indoor voice" to "speak up."
--Misty