High Sierra Airplay Problems I've been using a Mac running Sierra with an Airplay 2 capable Denon home cinemas amp for several months without any major problems.  

After upgrading to High Sierra I've run into two VERY annoying issues: 


Whenever I switch audio output from internal speaker to the Airplay amp and then hit play in a Youtube video or other video player iTunes starts instead of the video playing!  I have to quit iTunes and hit play again before it actually starts playing
Whenever playback starts all open network connections drop.  


Does anybody know how to stop iTunes autostarting when I switch to Airplay as the output device, and how to stop the connections from dropping?  Neither of these things happened in Sierra.  

EDIT: I tried getting a list of audio devices and AirPlay doesn't show up at all.  


  system_profiler SPAudioDataType Audio:

Devices:

    Built-in Output:

      Default Output Device: Yes
      Default System Output Device: Yes
      Manufacturer: Apple Inc.
      Output Channels: 2
      Current SampleRate: 44100
      Transport: Built-in
      Output Source: Internal Speakers

    iShowU Audio Capture:

      Default Input Device: Yes
      Input Channels: 2
      Manufacturer: shinywhitebox ltd.
      Output Channels: 2
      Current SampleRate: 44100
      Transport: Unknown
      Input Source: Default
      Output Source: Default

    Stream/Record:

      Manufacturer: Apple Inc.
      Output Channels: 2
      Current SampleRate: 44100
      Transport: Unknown
      Output Source: Default



However "Denon AVR-6400H" shows up as an available output device in the audio prefs pane and in the icon menu options.
That's probably because High Sierra introduced AirPlay2 (before, you were running AirPlay). With AirPlay2, it's no more a system wide output that's being redirected to your remote player, but instead a per-application output. Technically speaking, it's now a userspace library that application must implement.

Since only iTunes supports AirPlay2 for now, I guess this is the reason why it's being launched.

As to how to disable this, please check if your player can disable its AirPlay2 announce or use iTunes...