There are two really important principles to remember and live by in this passage. The first is, "It is written." Having Jesus remark that the written Word of God (Logos) is to be read, understood, believed and acted upon is a real message to you and me. The written Word is the preserved voice of God for all generations to teach us the truths that save our soul and give us the ability to live a life in God's will. Unlike the voice of God, which is subjective and constantly being questioned by the unbelieving, unfaithful and poorly taught Christians, the written Word of God is irreproachable, unchanging and indisputable. "It is written" should be the clearest statement concerning the infallibility of scripture. It should also be noted that, rather than always 'looking for a Word from God' that fits what you want isn't as biblically sound as looking to the Word to see exactly what God wants you to want. If the cultural/worldly Christian new the scriptures as well as they know their favorite sitcom, there would be no cultural/worldly Christians.
The second principle is, "Man shall not live by bread alone, but by every Word that proceedeth out of the mouth of God." We know from experience that the written Word of God is sure and tested and trustworthy. The voice of God, on the other hand, as you and I have both seen and experienced is not as easily discerned. Yet we are admonished to live by every Word that proceeds out of the mouth of God. It's really not that complicated.
It is true that God speaks to us. God directs us and teaches us and communicates with us and we should be very open to this kind of communication and we should be very open when others speak a word of wisdom. God quite often speaks to us through others. But I believe there is one qualifier that can help us shake out what is from God and what is just man's silly and unsubstantiated philosophies.
Can what was heard or said be substantiated from the written Word?