I want to add some Velocity controll to my CharacterController

Below I will post my Playercontroller script. In there I have a velocity variable that will controll the players movement.
I want to be able to make plattforms and other such things that will make my player fly up in the air when they interact and such.

To test the mechanics I am trying to add velocity when pressing jump, however; this is not taking effect.
The reason is because the velocity is reset to my “Horizontal / Vertical” * movementSpeed. How would I best go about on fixing this?

Sorry if I am unclear; I have a terrible headache and my main language is Swedish.

using UnityEngine;
using System.Collections;

[RequireComponent (typeof(CharacterController))]
public class FirstPersonController : MonoBehaviour {
	// Public variables
	public float movementSpeed = 10.0f;
	public float jumpSpeed = 10.0f;
	public float mouseSensitivity = 1.0f;
	public float maxHeadTilt = 75.0f;

	// Private Variables
	private Vector3 velocity = new Vector3(0,0,0);
	private float verticalRotation = 0.0f;
	private Vector3 movement = new Vector3(0,0,0);

	// Private Objects
	private CharacterController cc = null;
	private Camera _camera = null;

	// Use this for initialization
	void Start () {
		cc = GetComponent<CharacterController> ();
		_camera = GetComponentInChildren<Camera> ();
	// Update is called once per frame
	void Update () {

		// Velocity
		velocity.x = Input.GetAxis ("Horizontal") * movementSpeed;
		velocity.y += Physics.gravity.y * Time.deltaTime;
		velocity.z = Input.GetAxis ("Vertical") * movementSpeed;

		// Inputs
		if (Input.GetButtonDown ("Jump")) 
			velocity.z = jumpSpeed;
			velocity.y = jumpSpeed;

		// MouseLook
		transform.eulerAngles += new Vector3(0, Input.GetAxis("Mouse X") * mouseSensitivity, 0);
		verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
		verticalRotation = Mathf.Clamp (verticalRotation, -maxHeadTilt, maxHeadTilt);
		_camera.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);

		// Movement
		movement = new Vector3 (velocity.x, velocity.y, velocity.z);
		movement = transform.rotation * movement;
		cc.Move(movement * Time.deltaTime);
		Debug.Log (velocity);

Your moveSpeed is the same as your jumpSpeed, you wouldn’t notice any difference.
if you were to have

velocity.z += jumpSpeed;

instead of

velocity.z = jumpSpeed;

then it should work.