Well, I looked it up in Uriel Weinreich's Modern English-Yiddish/Yiddish English Dictionary, and couldn't find a Yiddish word that resembles Dona or Donna. My Hebrew dictionary disappeared some time ago, but a search of some online sites also didn't turn up any Hebrew word...the closest match was a word that transliterates as "Donah"...which is the Hebrew version of the name "Donna"! So I'll stick with the theory that it's a form of "la-da-da", unless someone comes up with a more plausible explanation. Hebrew and Yiddish songs often use those kinds of "nonsense" syllables...especially among the Hasidim, where wordless melody is central to the tradition.

Aloha,
Mark