As for sound, the addon and Kodi are independent.
You can use HDMI for Kodi, and the headphone jack (or another sound card) for the addon. It is what I do.
The onboard sound interface routes playback to either the jack or HDMI, but not to both.
Auto detection can be summarized like this: if, when the RPi boots, the display claims to have speakers, sound is output via HDMI by default; if not, it is output via the headphone jack. Auto detection is not always acurrate.
why is it not possible for the addon to send sound to HDMI and headphone, like kodi can do it? Is it to difficult to code?
For my case it is enough to get the sound over headphone, so i am only interested in the reason.