Example script to fade in an audio player when entering a trigger. There should be an AudioSource component also attached to this game object, and it should probably be in "loop" mode.

TriggerSound.cs

using UnityEngine;
using System.Collections;

public class TriggerSound : MonoBehaviour {
	float targetVolume = 0;
	
	void Start() {
		audio.Play();
	}
	
	void Update() {
		audio.pitch = Mathf.MoveTowards(audio.volume, targetVolume,
			Time.deltaTime);
	}
	
	void OnTriggerEnter() {
		targetVolume = 1;
	}
	
	void OnTriggerExit() {
		targetVolume = 0;
	}
}