What's the smallest organism (animal, insect, or any living creature) that can produce voice. And not just make sound, but has an organ to specifically to create sound, like how us mamals have vocal chords.
I will go with lesser water boatman(Micronecta scholtzi).
The loudest of everything as well as small.
Size: 0.07-inch (2-millimeter)
Source and Image Courtesy:
If we consider voice, I will go with Paedophryne amauensis
I'm putting my bets on the Pistol Shrimp (Alpheus heterochaelis) They can be grow 3cm to 5cm Long which is incredibly small for the amount of db it can create.
Their sound is a by-product of their large claw snapping shut and creating a cavitation bubble that creates a lot of pressure and then produces up to a recorded 218 Decibels. This "click" or "pop" of the bubble only lasts less than 1 millisecond. It also produces around 5000k in heat from the popping of the bubble.
(According to the Daily Mail which I honestly may say is probably not the most trustworthy source of accurate information http://www.dailymail.co.uk/sciencetech/article-1085398/Deadly-pistol-shrimp-stuns-prey-sound-loud-Concorde-UK-waters.html
Wikipedia Article : http://en.wikipedia.org/wiki/Alpheidae )