I have tried the HomeKit App on iPhone, iPad (both running 16.3.1) and Mac OS Ventura (13.2.1), and the settings on the Apple TV itself. Nowhere am I able to set the default speaker for the Apple TV to the HomePod mini. No such option shows up in the Apple TV settings in HomeKit anywhere, and the Apple TV HD setting show the TV speakers as the default audio output, with no opportunity to change it. Am I missing something? Also, sometimes the Apple TV retains the previous selection of the HomePod mini as audio output, and other times, it defaults back to the TV speakers and I have to re-select it in HomeKit on iPad or iPhone. Curiously, no ability to select the audio output for the Apple TV HD in HomeKit on Mac OS Ventura.