AudioEvent.cs

public class AudioPlayer : MonoBehaviour {
  [SerializedField]
  private float initialVolume;
  
  void Start() {
    Debug.Log(volume);
  }
}

public class AudioEventAsset : ScriptableObject {
  public float volume;
}

public class AudioSpawner : MonoBehaviour {
  public AudioEventAsset[] events;
  
  void Start() {
    foreach (var audioEvent in events) {
      var player = AddComponent<AudioPlayer>();
      player.initialVolume = events.volume; // エラー！privateメンバへアクセス
    }
  }
}