Where does the Bible say that nature will speak the word if men don't? I am trying to remember a verse I've heard, and almost positive I've read, before. 

I do not remember the exact phrasing. 

It was similar to: 'If no one is there to witness, even the rocks of the field and [trees? mountains? nature?] will witness/speak the word`. I believe this is in the New Testament, possibly Jesus speaking.

What is the passage that relates to what would happen if no one was around to witness?
I believe you are thinking of Luke 19:40:


  ‘I tell you [Pharisees],’ he [Jesus] replied, ‘if they [His disciples] keep quiet, the stones will cry out.’


Other passages that refer to the witness of creation include Psalm 19 and Romans 1:20.