Why won't my car start?

Cause you need to use Input.Get*Button*Down("Return") if there is such input axis name(Edit -> Project Settings -> Input) or Input.GetKeyDown(KeyCode.Return) Hope this helps! About the sound: It seems you have no object with "Audio Listener" script on your scene so you can't hear anything...

Found out that java is kind of glitchy in many ways. I rebuild the code again in C and it's working beautifully.

Well, by beautifully I mean there's no steering visually or suspension travel visually, but it works!

 using UnityEngine;
 using System.Collections;
 
 public class VolvoController : MonoBehaviour {
 
     public float EngineRPM = 0;
     public float Ignition = 0;
 
     public float BrakeAxis = 0;
     public float BrakeStrength = 500;
 
     public int Limiter = 1;
     public float Transfer = 0;
     public float GearRatio = 0;
     public float FinalDrive = 3.7f;
     public float Throttle = 0;
     public float RoadRPM = 0;
 
     public float TorqueMult = 1;
     public float RedLineRPM = 6500;
     public float IdleRPM = 800;
     public float StallRPM = 400;
     public float EngineFriction = 0.01f;
 
     public AudioClip EngineStart;
     public AudioClip EngineSound;
     public AudioClip EngineStop;
 
     public WheelCollider WheelFL;
     public WheelCollider WheelFR;
     public WheelCollider WheelRL;
     public WheelCollider WheelRR;
 
     public Transform WheelFLTrans;
     public Transform WheelFRTrans;
     public Transform WheelRLTrans;
     public Transform WheelRRTrans;
 
     // Update is called once per frame
     void Update () {
         //ENGINE
         if (Input.GetKeyDown(KeyCode.Return)) {
             if (EngineRPM < StallRPM){
                 Ignition = 2;
             }
         }
         if (Input.GetKeyUp(KeyCode.Return)){
             Ignition = 1;
         }
 
         if (Input.GetKey(KeyCode.K)){
             if (Ignition >= 1){
                 //EngineSound.Stop();
                 //EngineStop.Play();
             }
             Ignition = 0;
         }
 
         if (EngineRPM > RedLineRPM){
             Limiter = 0;
         }
         
         if (EngineRPM < RedLineRPM - 200){
             Limiter = 1;
         }
         
         if (Input.GetAxis ("Vertical") > 0) {
             Throttle = Input.GetAxis ("Vertical");
         } else if (Input.GetAxis ("Vertical") <= 0) {
             Throttle = 0;
         }
 
         float EngineTorque = TorqueMult * 250 + (EngineRPM * EngineFriction) ;
 
         if (EngineRPM > StallRPM){
             EngineRPM = EngineRPM + EngineTorque * Limiter * Throttle ;
         }
         
         if (EngineRPM < IdleRPM && EngineRPM > StallRPM && Ignition >= 1){
             EngineRPM = EngineRPM + 35;
         }
         
         if (EngineRPM > 0){
             EngineRPM = EngineRPM - (EngineRPM * EngineFriction);
         }
         
         if (EngineRPM < StallRPM){
             EngineRPM = EngineRPM - 25;
         }
         
         if (Ignition == 2){
             if (EngineRPM < StallRPM){
                 EngineRPM = EngineRPM + 50;
             }
         }
 
         if (Ignition == 1 && EngineRPM > StallRPM){
             //EngineStart.Stop();
             //EngineSound.Play();
         }
         
         if (EngineRPM < 0){
             EngineRPM = 0;
         }
 
         //TRANSMISSION
         if (Input.GetKeyDown(KeyCode.Alpha1)){
             GearRatio = 4;
         }
         if (Input.GetKeyDown(KeyCode.Alpha2)){
             GearRatio = 3;
         }
         if (Input.GetKeyDown(KeyCode.Alpha3)){
             GearRatio = 2;
         }
         if (Input.GetKeyDown(KeyCode.Alpha4)){
             GearRatio = 1;
         }
         if (Input.GetKeyDown(KeyCode.Alpha5)){
             GearRatio = 0.7f;
         }
         if (Input.GetKeyDown(KeyCode.N)){
             GearRatio = 0;
         }
         if (Input.GetKeyDown(KeyCode.R)){
             GearRatio = -3;
         }
 
         RoadRPM = (WheelFL.rpm + WheelFR.rpm) * 0.5f * GearRatio * FinalDrive;
         
         Transfer = (EngineRPM - RoadRPM);
 
         if (EngineRPM > StallRPM && GearRatio != 0) {
             EngineRPM = EngineRPM - Transfer * EngineRPM / RedLineRPM * 0.5f;
         }
         WheelFR.motorTorque = 0.05f * Transfer * GearRatio * FinalDrive;
         WheelFL.motorTorque = 0.05f * Transfer * GearRatio * FinalDrive;
 
         //BRAKES
         if (Input.GetAxis("Vertical") < 0) {
             BrakeAxis = -Input.GetAxis("Vertical");
         }
         else if (Input.GetAxis("Vertical") >= 0){
             BrakeAxis = 0;
         }
 
         WheelFL.brakeTorque = BrakeStrength * BrakeAxis;
         WheelFR.brakeTorque = BrakeStrength * BrakeAxis;
         WheelRL.brakeTorque = BrakeStrength * BrakeAxis;
         WheelRR.brakeTorque = BrakeStrength * BrakeAxis;
 
         //MISC
         if (Ignition >= 1){
             WheelFL.steerAngle = 40 * Input.GetAxis("Horizontal");
             WheelFR.steerAngle = 40 * Input.GetAxis("Horizontal");
         }
 
         WheelFLTrans.Rotate(WheelFL.rpm / 60 * 360 * Time.deltaTime, 0, 0);
         WheelFRTrans.Rotate(WheelFR.rpm / 60 * 360 * Time.deltaTime, 0, 0);
         WheelRLTrans.Rotate(WheelRL.rpm / 60 * 360 * Time.deltaTime, 0, 0);
         WheelRRTrans.Rotate(WheelRR.rpm / 60 * 360 * Time.deltaTime, 0, 0);
         
         WheelFL.transform.rotation = Quaternion.Euler(transform.rotation.eulerAngles.x, WheelFL.steerAngle, transform.rotation.eulerAngles.z);
         WheelFR.transform.rotation = Quaternion.Euler(transform.rotation.eulerAngles.x, WheelFR.steerAngle, transform.rotation.eulerAngles.z);
     }
 }