And all the people saw the voices (Exodus 20:15)
They saw what is ordinarily heard and they heard what is ordinarily seen.
(Midrash Lekach Tov; Rashi)
As physical beings, we "see" physical reality. On the other hand, G-dliness
and spirituality is only something that is "heard" -- it can be
discussed, perhaps even assimilated to some extent, but not experienced first
At the revelation at Sinai, we "saw what is ordinarily heard" -- we experienced
the divine as an immediate, tangible reality. On the other hand, that which is
ordinarily "seen" -- the material world -- was something merely
"heard," to be accepted or rejected at will.
(The Lubavitcher Rebbe)