fixed #490; made add_audio.sh more robust
fixed #490; made add_audio.sh more robust

Tatoeba/tatoeba2,shashank19gaurav/tatoeba2,shashank19gaurav/tatoeba2,shashank19gaurav/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,shashank19gaurav/tatoeba2