AudioSource.pitch  

// Decreases the pitch in the given seconds

var startingPitch = 4;
var timeToDecrease = 5;

function Start() {
    audio.pitch = startingPitch;
}

function Update() {
    if(audio.pitch > 0)
        audio.pitch -= ((Time.deltaTime * startingPitch) / timeToDecrease);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public int startingPitch = 4;
    public int timeToDecrease = 5;
    void Start() {
        audio.pitch = startingPitch;
    }
    void Update() {
        if (audio.pitch > 0)
            audio.pitch -= Time.deltaTime * startingPitch / timeToDecrease;
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public startingPitch as int = 4

    public timeToDecrease as int = 5

    def Start():
        audio.pitch = startingPitch

    def Update():
        if audio.pitch > 0:
            audio.pitch -= ((Time.deltaTime * startingPitch) / timeToDecrease)