esphome:
  on_boot:
    then:
      - router.speaker.switch_output:
          id: router_id
          target_speaker: speaker_b_id
      # id omitted: auto-resolved since there's a single router instance
      - router.speaker.switch_output:
          target_speaker: !lambda return id(speaker_a_id);

speaker:
  - platform: i2s_audio
    id: speaker_a_id
    i2s_audio_id: i2s_audio_bus
    dac_type: external
    i2s_dout_pin: ${a_dout_pin}
    sample_rate: 48000
    bits_per_sample: 16bit
    channel: stereo
  - platform: i2s_audio
    id: speaker_b_id
    i2s_audio_id: i2s_audio_bus
    dac_type: external
    i2s_dout_pin: ${b_dout_pin}
    spdif_mode: true
    use_apll: true
    sample_rate: 48000
    bits_per_sample: 16bit
    channel: stereo
    i2s_mode: primary
  - platform: router
    id: router_id
    output_speakers:
      - speaker_a_id
      - speaker_b_id
    sample_rate: 48000
    bits_per_sample: 16
    num_channels: 2