Unity5 Vehicles package sound problem

i was trying to use the default car from the car package that comes with unity but whenever i run the game there is no engine sound and i see this error: NullReferenceException: Object reference not set to an instance of an object UnityStandardAssets.Vehicles.Car.CarAudio.Update () (at Assets/Standard Assets/Vehicles/Car/Scripts/CarAudio.cs:90)

i can hear the sound when i brake but that's it. i also looked at line 90 in the code and what it says in the error is not what is on line 90. somebody please help!

Car Audio script:

 using System;
 using UnityEngine;
 using Random = UnityEngine.Random;
 
 namespace UnityStandardAssets.Vehicles.Car
 {
     [RequireComponent(typeof (CarController))]
     public class CarAudio : MonoBehaviour
     {
         // This script reads some of the car's current properties and plays sounds accordingly.
         // The engine sound can be a simple single clip which is looped and pitched, or it
         // can be a crossfaded blend of four clips which represent the timbre of the engine
         // at different RPM and Throttle state.
 
         // the engine clips should all be a steady pitch, not rising or falling.
 
         // when using four channel engine crossfading, the four clips should be:
         // lowAccelClip : The engine at low revs, with throttle open (i.e. begining acceleration at very low speed)
         // highAccelClip : Thenengine at high revs, with throttle open (i.e. accelerating, but almost at max speed)
         // lowDecelClip : The engine at low revs, with throttle at minimum (i.e. idling or engine-braking at very low speed)
         // highDecelClip : Thenengine at high revs, with throttle at minimum (i.e. engine-braking at very high speed)
 
         // For proper crossfading, the clips pitches should all match, with an octave offset between low and high.
 
 
         public enum EngineAudioOptions // Options for the engine audio
         {
             Simple, // Simple style audio
             FourChannel // four Channel audio
         }
 
         public EngineAudioOptions engineSoundStyle = EngineAudioOptions.FourChannel;// Set the default audio options to be four channel
         public AudioClip lowAccelClip;                                              // Audio clip for low acceleration
         public AudioClip lowDecelClip;                                              // Audio clip for low deceleration
         public AudioClip highAccelClip;                                             // Audio clip for high acceleration
         public AudioClip highDecelClip;                                             // Audio clip for high deceleration
         public float pitchMultiplier = 1f;                                          // Used for altering the pitch of audio clips
         public float lowPitchMin = 1f;                                              // The lowest possible pitch for the low sounds
         public float lowPitchMax = 6f;                                              // The highest possible pitch for the low sounds
         public float highPitchMultiplier = 0.25f;                                   // Used for altering the pitch of high sounds
         public float maxRolloffDistance = 500;                                      // The maximum distance where rollof starts to take place
         public float dopplerLevel = 1;                                              // The mount of doppler effect used in the audio
         public bool useDoppler = true;                                              // Toggle for using doppler
 
         private AudioSource m_LowAccel; // Source for the low acceleration sounds
         private AudioSource m_LowDecel; // Source for the low deceleration sounds
         private AudioSource m_HighAccel; // Source for the high acceleration sounds
         private AudioSource m_HighDecel; // Source for the high deceleration sounds
         private bool m_StartedSound; // flag for knowing if we have started sounds
         private CarController m_CarController; // Reference to car we are controlling
 
 
         private void StartSound()
         {
             // get the carcontroller ( this will not be null as we have require component)
             m_CarController = GetComponent<CarController>();
 
             // setup the simple audio source
             m_HighAccel = SetUpEngineAudioSource(highAccelClip);
 
             // if we have four channel audio setup the four audio sources
             if (engineSoundStyle == EngineAudioOptions.FourChannel)
             {
                 m_LowAccel = SetUpEngineAudioSource(lowAccelClip);
                 m_LowDecel = SetUpEngineAudioSource(lowDecelClip);
                 m_HighDecel = SetUpEngineAudioSource(highDecelClip);
             }
 
             // flag that we have started the sounds playing
             m_StartedSound = true;
         }
 
 
         private void StopSound()
         {
             //Destroy all audio sources on this object:
             foreach (var source in GetComponents<AudioSource>())
             {
                 Destroy(source);
             }
 
             m_StartedSound = false;
         }
 
 
         // Update is called once per frame
         private void Update()
         {
             // get the distance to main camera
             float camDist = (Camera.main.transform.position - transform.position).sqrMagnitude;
 
             // stop sound if the object is beyond the maximum roll off distance
             if (m_StartedSound && camDist > maxRolloffDistance*maxRolloffDistance)
             {
                 StopSound();
             }
 
             // start the sound if not playing and it is nearer than the maximum distance
             if (!m_StartedSound && camDist < maxRolloffDistance*maxRolloffDistance)
             {
                 StartSound();
             }
 
             if (m_StartedSound)
             {
                 // The pitch is interpolated between the min and max values, according to the car's revs.
                 float pitch = ULerp(lowPitchMin, lowPitchMax, m_CarController.Revs);
 
                 // clamp to minimum pitch (note, not clamped to max for high revs while burning out)
                 pitch = Mathf.Min(lowPitchMax, pitch);
 
                 if (engineSoundStyle == EngineAudioOptions.Simple)
                 {
                     // for 1 channel engine sound, it's oh so simple:
                     m_HighAccel.pitch = pitch*pitchMultiplier*highPitchMultiplier;
                     m_HighAccel.dopplerLevel = useDoppler ? dopplerLevel : 0;
                     m_HighAccel.volume = 1;
                 }
                 else
                 {
                     // for 4 channel engine sound, it's a little more complex:
 
                     // adjust the pitches based on the multipliers
                     m_LowAccel.pitch = pitch*pitchMultiplier;
                     m_LowDecel.pitch = pitch*pitchMultiplier;
                     m_HighAccel.pitch = pitch*highPitchMultiplier*pitchMultiplier;
                     m_HighDecel.pitch = pitch*highPitchMultiplier*pitchMultiplier;
 
                     // get values for fading the sounds based on the acceleration
                     float accFade = Mathf.Abs(m_CarController.AccelInput);
                     float decFade = 1 - accFade;
 
                     // get the high fade value based on the cars revs
                     float highFade = Mathf.InverseLerp(0.2f, 0.8f, m_CarController.Revs);
                     float lowFade = 1 - highFade;
 
                     // adjust the values to be more realistic
                     highFade = 1 - ((1 - highFade)*(1 - highFade));
                     lowFade = 1 - ((1 - lowFade)*(1 - lowFade));
                     accFade = 1 - ((1 - accFade)*(1 - accFade));
                     decFade = 1 - ((1 - decFade)*(1 - decFade));
 
                     // adjust the source volumes based on the fade values
                     m_LowAccel.volume = lowFade*accFade;
                     m_LowDecel.volume = lowFade*decFade;
                     m_HighAccel.volume = highFade*accFade;
                     m_HighDecel.volume = highFade*decFade;
 
                     // adjust the doppler levels
                     m_HighAccel.dopplerLevel = useDoppler ? dopplerLevel : 0;
                     m_LowAccel.dopplerLevel = useDoppler ? dopplerLevel : 0;
                     m_HighDecel.dopplerLevel = useDoppler ? dopplerLevel : 0;
                     m_LowDecel.dopplerLevel = useDoppler ? dopplerLevel : 0;
                 }
             }
         }
 
 
         // sets up and adds new audio source to the gane object
         private AudioSource SetUpEngineAudioSource(AudioClip clip)
         {
             // create the new audio source component on the game object and set up its properties
             AudioSource source = gameObject.AddComponent<AudioSource>();
             source.clip = clip;
             source.volume = 0;
             source.loop = true;
 
             // start the clip from a random point
             source.time = Random.Range(0f, clip.length);
             source.Play();
             source.minDistance = 5;
             source.maxDistance = maxRolloffDistance;
             source.dopplerLevel = 0;
             return source;
         }
 
 
         // unclamped versions of Lerp and Inverse Lerp, to allow value to exceed the from-to range
         private static float ULerp(float from, float to, float value)
         {
             return (1.0f - value)*from + value*to;
         }
     }
 }