Parshat Yitro

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 hand.
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)