מדרש על יחזקאל 10:5

11) Similarly, (Ezekiel 10:5): "And the sound of the wings of the cherubs was heard (only) until the outer court." I might think that this was because it was a low sound; it is, therefore, written (Ezekiel 10:5): "like the voice of the almighty G d in His speaking" — in His speaking at Sinai. If so, why (only) "until the outer court"? Once it reached the outer court, it was "cut off."