According to Aristophanes' comedy Batrachoi ("The Frogs", first presented at the Lenaea in 405 BCE), this is the sound made by frogs:
Brekekekex koax koax,
Brekekekex koax koax,
Marshy children of the waters,
the harmonious cry of hymns,
Let us sing, my sweet
song, Koaxkoax,
which for Nysian
Dionysos, son of Zeus,
we sang at Limnae
when in drunken revelry
at the Feast of the Jars
the crowd of people marches to my sanctuary.
Brekekekex koax koax,
- as translated by Matthew Dillon
The phrase was also used by Hans Christian Andersen, in the fairy tale Tommeliden ("Thumbelina", 1835), probably to show off Andersen's classical education (he was generally very self-absorbed).