Adjusting the x and y velocity of the ball.

Ball.cs

using UnityEngine;
using System.Collections;

public class Ball : MonoBehaviour {
	public float margin = 1.5f; // don't want to be too controlling
	public float factor = 100f;

	private Paddle paddle;
	private bool hasStarted = false;
	private Vector3 paddleToBallVector;
	
	private Rigidbody2D rb;
	private float velocity = 6f;
	private float speed;
	private float dir;


	// Use this for initialization
	void Start () 
	{
		rb = GetComponent<Rigidbody2D> ();
		paddle = GameObject.FindObjectOfType<Paddle>();
		paddleToBallVector = this.transform.position - paddle.transform.position;
	}
	
	// Update is called once per frame
	void Update () 
	{
		if (!hasStarted) 
		{
			// lock ball to paddle
			this.transform.position = paddle.transform.position + paddleToBallVector;

			// wait for mouse click to launch
			if (Input.GetMouseButtonDown (0)) 
			{
				print ("mouse clicked, launch ball");
				hasStarted = true;
				this.rigidbody2D.velocity = new Vector2 (velocity, velocity);
			}
		}
	}

	void OnCollisionEnter2D (Collision2D other) 
	{
		if (hasStarted) {audio.Play ();

			// adjust x: speed up , slow down
			speed = Mathf.Abs (rb.velocity.x);
			dir = Mathf.Sign (rb.velocity.x);
			if (speed  < velocity - margin) {rb.AddForce (new Vector2 (dir * factor, 0));}
			if (speed  > velocity + margin) {rb.AddForce (new Vector2 (-dir * factor, 0));}

			// adjust y: speed up , slow down
			speed = Mathf.Abs (rb.velocity.y);
			dir = Mathf.Sign (rb.velocity.y);
			if (speed < velocity - margin) {rb.AddForce (new Vector2 (0, dir * factor));}
			if (speed > velocity + margin) {rb.AddForce (new Vector2 (0, -dir * factor));}
		
		}

	}
	
}