@script RequireComponent(AudioSource)
function Start() {
    audio.volume = 0.2;
}

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
public class example : MonoBehaviour {
    void Start() {
        audio.volume = 0.2F;
    }
}

import UnityEngine
import System.Collections

[RequireComponent(AudioSource)]
class example(MonoBehaviour):

    def Start():
        audio.volume = 0.2F