You should go through this thread.
Discussion - Audio issues on XStream Smart box
In a nutshell, Xstream Box doesn't produce Dolby on Prime Video app. Netflix, hotstar and HD channels like Star Movies HD works fine and produces Dolby. However, with the recent update, after the manual audio mode was removed, several members reported issues with Dolby audio, even on these. There's something wrong with the update, please report it on Twitter @Airtel_Presence and they can possible roll back your STB to the previous version.