Was the 1883 Krakatoa explosion the loudest sound ever heard by humans on earth?I read that the sound was heard over 2000 miles away.

As far as we know, yes, it was the loudest noise heard on earth in the historic period, ie in the past six to seven thousand years.  The fourth explosion on 27 August, 1883 was recorded on barographs around the world for five days, the sound wave circling the globe seven times.  It was heard as far away as the Island of Rodrigues, near Mauritius some 3,000 miles away.  The final explosion was equivalent to 200 megatons of TNT, some 13 times larger than the Hiroshima bomb.  The largest nuclear device ever detonated was the Tsar Bomba, of some 50 megatons.  It is estimated that atmospheric effects occured over 70% of the earth, and colorful sunsets and moonrises caused by atmospheric dust lasted nearly a decade.  Tsunamis and pyroclastic flows caused approximately 36,417 deaths.

Krakatau has rebuilt itself and exploded repeatedly throughout history, in 1680, 416 and several other times.  Krakatau has been the most dangerous volcano throughout human history, and some vulcanologists believe it was once the site of the greatest volcanic eruption the earth has seen.