What's the smallest organism that can produce voice?
I will go with lesser water boatman(Micronecta scholtzi).

The loudest of everything as well as small.

Size: 0.07-inch (2-millimeter)


Produces the highest sound by size ratio.
It's sound is almost equivalent to pounding jackhammer at two feet distance.


http://www.plosone.org/article/fetchObject.action?uri=info:doi/10.1371/journal.pone.0021089.g001&amp;representation=PNG_L

Source and Image Courtesy: 

So Small, So Loud: Extremely High Sound Pressure Level from a Pygmy Aquatic Insect (Corixidae, Micronectinae)

Youtube: https://www.youtube.com/watch?v=i7-hREXmMFM

Update:

If we consider voice, I will go with Paedophryne amauensis


Smallest frog an known vertebrate 
size: 7.7 mm (0.30 in)