How to build and use SpeakerGroups

General
3

I have found a solution to implement this with HomeAssistant:

media_player:
- platform: universal
  name: Lautsprechergruppe (Wohnzimmer)
  unique_id: lautsprechergruppe_wohnzimmer
  children:
    - media_player.wohnzimmer
    - media_player.hegel_h390
  commands:
    turn_off:
      action: media_player.turn_off
      data:
        entity_id: 
          - media_player.wohnzimmer
    turn_on:
      action: media_player.turn_on
      data:
        entity_id: 
          - media_player.wohnzimmer
    volume_up:
      action: media_player.volume_set
      data:
        volume_level: "{{states.media_player.hegel_h390.attributes.volume_level + 0.01}}"
      target:
        device_id:
          - cf7c35b47ff043d1235935f325a2cc4a
          - d29d51c10faa27e1d57199e4c426c4b6
    volume_down:
      action: media_player.volume_set
      data:
        volume_level: "{{states.media_player.hegel_h390.attributes.volume_level - 0.01}}"
      target:
        device_id:
          - cf7c35b47ff043d1235935f325a2cc4a
          - d29d51c10faa27e1d57199e4c426c4b6
    volume_set:
      action: media_player.volume_set
      target:
        entity_id: 
          - media_player.wohnzimmer
          - media_player.hegel_h390
      data:
        volume_level: "{{ volume_level }}"
  attributes:
    is_volume_muted: media_player.wohnzimmer|is_volume_muted
    state_template: media_player.wohnzimmer|state
    volume_level: media_player.hegel_h390|volume_level
    media_content_type: states.media_player.wohnzimmer|media_content_type
    media_duration: states.media_player.wohnzimmer|media_duration
    media_position: states.media_player.wohnzimmer|media_position
    media_position_updated_at: states.media_player.wohnzimmer|media_position_updated_at
    source: states.media_player.wohnzimmer|source
    sound_mode: states.media_player.wohnzimmer|sound_mode
    entity_picture: "{{states.media_player.wohnzimmer.attributes.entity_picture}}"
    media_title: "{{states.media_player.wohnzimmer.attributes.media_title}}"
    media_album_name: "{{states.media_player.wohnzimmer.attributes.media_album_name}}"
    media_artist: "{{states.media_player.wohnzimmer.attributes.media_artist}}"
  device_class: receiver

This media player can then be transferred to the WebConfigurator. The only thing missing here is the progress bar. But it works. I can also use it to control the volume of both devices together.