OK, so, if I understand right, it works on certain boot but not on others? consistently, as in if you have problems it will be constently from the start?
... I'm not sure if that will help, but it won't hurt to ensure you force the audio through HDMI with:
curl https://get.pimoroni.com/audio | bash
and choose option 3.
That said I'm not sure that is your problem, but that is the only thing I can think of if behaviour vary from boot to boot.