The VAV project using audio files for sentences from tatoeba.org Most of the audio for the sentences are by manythings.org
The project using audio files for vocabulary from gstatics.com
German audio pronunciations taken from de.langenscheidt.com
The project using babylon online dictionary as embedded for more details about words.
English word popularity, calculated based on the data was given on "TV and movie scripts" section at Wiktionary:Frequency lists