Just a thought.
Have you looked at the Android settings (not the Humax Aura settings, which are overlayed and specifically designed for operating the Aura)?
At the operating system level of Android there are usually options for HDMI output. Mine has Auto, LPCM and Raw. Raw sends the audio signal directly without any processing, so that whatever device it goes into can do the work.
If you don't already have access to the OS settings, there are numerous apps available that make this access possible.