Nonverbal spells

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No: HPFGUIDX 135589

I was wondering what spells are cast nonverbally, and what spells are 
cast verbally.  There, obviously, is a big emphasis on non-verbal 
spells in HBP, and I noticed that even though most incantations 
are "thought", Dumbledore says "Lumos."  The unforgivable curses are 
also spoken.  However, in the Ministry Battle in OoP, the Death Eaters 
were using a lot of non-verbal, as were the Oop memebers.  Why, if all 
spells could be non-verbal, would anybody speak the incantation out 
loud?  This seems like unneccessary effort to me. 

Any ideas?

- Sarah