Is it permissible and recommended to say "Audhubillah" after Takbir but before Bismillah at the beginning of the prayer? If so, should it be said in a whispering voice?
Yes it is recommended to say أعوذ بالله من الشيطان الرجيم after Takbiratul Ihraam before starting the Sura of Al-Hamd, with whispering voice.
Wassalam.