Sondeckis was born in Šiauliai in 1928. He graduated from the Lithuanian Academy of Music and Theatre (Vilnius Conservatory) as a violinist with Alexander Livontas in 1952 and studied conducting with Igor Markevich.

Sondeckis was a member of the Department of String Instruments at the Lithuanian SSR State Conservatoire. Sondeckis began conducting in 1955 and started at the Student Orchestra of the National M.K.