（スクリプト）

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class TankMovement_1 : MonoBehaviour
{
    private float moveS;
    public float turnS;
    private Rigidbody rb;
    private float movementInputValue;
    private float turnInputValue;

    // ★音の切り替え
    public AudioSource audioSource;
    public AudioClip[] souds;

    private void Awake()
    {
        Cursor.lockState = CursorLockMode.Locked;
    }

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {
        TnakMove();
        TankTurn();

        // ★音の切り替え
        EngineSound();
    }

    void TnakMove()
    {
        movementInputValue = Input.GetAxis("Vertical");
        Vector3 movement = transform.forward * movementInputValue * moveS * Time.deltaTime;
        rb.MovePosition(rb.position + movement);
        var inputValue = movementInputValue * movementInputValue;

        if(inputValue == 1)
        {
            moveS += Time.deltaTime * 2;
            print(moveS);

            if(moveS > 7)
            {
                moveS = 7;
            }
        }

        if(inputValue == 0)
        {
            moveS = 0;
        }
    }

    void TankTurn()
    {
        turnInputValue = Input.GetAxis("Horizontal");
        float turn = turnInputValue * turnS * Time.deltaTime;
        Quaternion turnRotation = Quaternion.Euler(0, turn, 0);
        rb.MoveRotation(rb.rotation * turnRotation);
    }

    // ★音の切り替え
    void EngineSound()
    {
        // Mathf.Absは絶対値を返す
        if (Mathf.Abs(movementInputValue) < 0.1f && Mathf.Abs(turnInputValue) < 0.1f)
        {
            if (audioSource.clip == souds[1])
            {
                // アイドリング音に変更
                audioSource.clip = souds[0];
                audioSource.Play();
            }
        }
        else
        {
            if (audioSource.clip == souds[0])
            {
                // 走行音に変更
                audioSource.clip = souds[1];
                audioSource.Play();
            }
        }
    }
}

（設定）