Even from the very fist lesson students can start getting an imagination of sound from the notes on the page. The Frances Clark lesson books begin with 2 simple songs - one going up and one going down. Sometimes the student and I will make random sounds of high and low to really imagine the sounds that are going to happen in the song, sometimes we talk like a teeny mouse or a big bear to imagine the high and low sounds. That way when they are practicing they can have a way to double-check that they are making the correct sounds on the piano.
Imagining what you are seeing is especially important for students who like to hear something first before they play it. I have had many students say "I can't play this song because I don't know how it sounds." And that's when we start imagining!! First we do just general high-ness and low-ness together with rhythm. Then after several weeks of doing that I will start to have them home in on the notes more exactly. Sometimes we will split up focusing on just rhythm and then just pitch. It is neat to see them getting more and more accurate as time goes by!
It is fun to see a student start being surprised when they can sing their music pretty accurately the first time they see it. It is a great boost to their practice confidence!