I found this very interesting TEDTALK which though primarily is something more about the sound of the speakers voice not directly related to English, I think there is a lot here that can be transposed into the classroom. Particularly good for students who have a problem with mumbling, incoherence, garbled messages and confidence. Julian Treasure is saying that it all comes down to your intention when you speak. Teaching this intention can be something we can do as ESL teachers too. Check it out.