[audio_dac] Add a hook that runs once the I2S clocks are running (#19900)

esphome/components/audio_dac/audio_dac.h

@@ -14,6 +14,10 @@ class AudioDac {
   virtual bool is_muted() = 0;
   virtual float volume() = 0;
 
+  /// Called from the main loop by the speaker driving this DAC once its audio clocks are running.
+  /// DACs that can only be configured while clocked (e.g. DSP state lost across clock stops) override this.
+  virtual void on_audio_started() {}
+
  protected:
   bool is_muted_{false};
 };

esphome/components/i2s_audio/speaker/i2s_audio_speaker.cpp

@@ -77,6 +77,12 @@ void I2SAudioSpeakerBase::loop() {
     ESP_LOGV(TAG, "Started");
     xEventGroupClearBits(this->event_group_, SpeakerEventGroupBits::TASK_RUNNING);
     this->state_ = speaker::STATE_RUNNING;
+#ifdef USE_AUDIO_DAC
+    // The task raises TASK_RUNNING only after enabling the channel, so the clocks are already running
+    if (this->audio_dac_ != nullptr) {
+      this->audio_dac_->on_audio_started();
+    }
+#endif  // USE_AUDIO_DAC
   }
   if (event_group_bits & SpeakerEventGroupBits::TASK_STOPPING) {
     ESP_LOGV(TAG, "Stopping");