trying to play sound when ball is grounded

Unity Engine
1

Hello friends, I am making a game where the player is a ball that rolls around. I have a problem making a script that makes a sound when the ball is rolling. Here is my script.

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
public class PlayRoll : MonoBehaviour {
    private bool grounded;


    void Start() {
   
        grounded = false;

    }

    void OnTriggerEnter(Collider other)
    {

        if (other.gameObject.CompareTag ("ground"))
        {

            grounded = true;

        }
            else
        {
                grounded = false;
            }

        playSoundWhenGrounded ();
      
}
    void playSoundWhenGrounded() {
       
        AudioSource audio = GetComponent<AudioSource>();
       
        if (grounded == true) {
            audio.Play();
           
        }
       
    }
}

also, if u know a way to make that sound only when the ball is gaining speed, so that it wont play this sound when is grounded but the ball is not moving, don’t hesitate to tell me :). Thank you!

2

You need an onCollisionLeave too, and set the grounded to false there.
The speeding up bit:

Pseudo(kinda) code:
Vector3 pos: pos at startup
float vBefore: 0

Update:
   float v: current pos - pos
   if v > vBefore
      play sound
   endIf
   vBefore = v
   pos = current pos
EndOfUpdate