[Bibio]

i know that you only use your HTPC for watching movies so my best suggestion would be to turn off the onboard sound device in your BIOS.

or right click the speaker icon on your taskbar and then choose playback devices from the list.
in the list you will find a device called "AMD High Definition Audio Device" (it may be called something else) with a icon of a monitor or box to represent your AVR this is usually your GPU.
you need to set your GPU as your "default device" sound device.

now start kodi and go to settings -> system -> audio and copy over the settings in the first screen capture above.

now go to settings -> system -> dsplayer -> filters "-audio decoder" and copy the settings in the second screen capture above. this is your LAV audio.

restart kodi and play a movie. on exit of playback you will find that it helps switching of audio on your AVR.

explanation:
you dont need HD audio while browsing the Kodi interface (GUI). if you set WASPI and HD audio then Kodi might 'lock' the HD audio on dsplayer exit.
this results in HD audio codecs still being displayed on your AVR display, sometimes it "unlocks" when you start to brows kodi again or after a small period of time has elapsed.
you are basically telling kodi to drop audio to stereo while browsing kodi. but when playing a movie the LAV audio takes over until exit of playback at which point kodi takes over again.