Input.deviceOrientation

public static DeviceOrientation deviceOrientation ;

Description

Device physical orientation as reported by OS. (Read Only)

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    AudioSource audioSource;

    void Start()
    {
        audioSource = GetComponent<AudioSource>();
    }

    void Update()
    {
        if (Input.deviceOrientation == DeviceOrientation.FaceDown)
            audioSource.Play();
    }
}