This is something I have never tried before but maybe the problem should be divided as two,

1. To identify if the system has identified the presence of audio on HDMI
2. To channel the voice to the right audio device

For (1), you can try this command 'aplay -l' to see if there is 'HDMI' audio
For (2), you need to run 'alsamixer' and/or 'amixer' to channel the audio to the HDMI.