There is King Pasenadi of Kosala who heard the voices of 4 beings trapped in a niraya, in his dreams. These 4 beings were humans at the time of Lord Buddha Kassapa. He was not able to fully understand what they were saying due to their agonies, however, he heard four Pali syllables and asked Lord Buddha for an explanation.Annatarapurisa Vatthu
Bāla Vagga V. 1. The King and the Poor Man with a Beautiful Wife Aññatarapurisavatthu (60)
The four syllables : ‘Du, Sa, Na, So.’