I'm undecided on these yet. I'm on day two, they are relatively comfortable, and I have no trouble hearing other people speaking, but my own voice sounds muffled to me and I'm not sure whether they are really muting the handpiece all that much. I need to play around with them a bit more.
Yes, the muffled sound of your own voice is an indicator that they are working! The brain compares the sound through the air (air conduction - outside sound) to the sound that travels in your skull (own generated sound). If it changes from the norm, then we call that occlusion, and the symptom is your own voice sounds dull or different. :)