I want to implement the UI slider and adjust the volume.

var url:URL?{get}

(data as well.) {get} means read-only and non-writeable properties ({getset} if writable)
In other words, if you replace the CD (if you change the URL or file path), it doesn't mean that another song will be played.Once you generate an instance by specifying the URL in Init(contentsOfurl:URL), you cannot change it later.

Based on the code provided, I do not believe that Swift and object-oriented instance references are correctly understood.It's a good opportunity, so let's study Swift's "class reference type."
First, Swift limits reference objects to classes only.Other structures, Enumeration, etc. are value types, not reference types.
So, what is a reference type? First, run the following code in Playground in Xcode.

//Defining Class Hoge
class Hoge {
    var name —String

    var volume —Float=0

    init(name:String){
        self.name = name
    }
}

// Instance class Hoge.
var hoge1 = Hoge(name: "Taro")
hoge1.volume=100
print(hoge1.volume)//100

var hoge0 = substituted a reference for hoge1//hoge1 for hoge0.
hoge0.volume=200
print(hoge1.volume)//200
// Both variables hoge0 and hoge1 have references to the same instance.

This is a reference-type technique in which operations (changes) to the variable hoge0 are reflected in the variable hoge1, but this alone might make you wonder, "Why?"However, considering that the reference type of this class can provide a mechanism that reflects user input on the ViewControllerA screen objects (for example, tables, collections, and so on).
At the same time, however, this mechanism means that if you substitute a reference for another class of instances for a variable that substitutes a reference for an instance of one class, you will no longer refer to an instance of the previous class.

class Hoge {
    var name —String

    var volume —Float=0

    init(name:String){
        self.name = name
    }
}

var hoge1 = Hoge(name: "Taro")
hoge1.volume=100
print(hoge1.volume)//100

var hoge2 = Hoge(name: "Hanako")
hoge2.volume=50
print(hoge2.volume)//50

The hoge1 = hoge2 // variable hoge1 is substituted by a reference to an instance of name = "Hanako" that is substituted by hoge2
print(hoge1.name)//Hanako
print(hoge1.volume)//50

I believe that the above attempts (or experiments) have shown that the code you provided does not work at all as intended.

Below, away from the class reference type, I would like to write practical programming advice as a free gift.
First, write the initialization process for the UIViewController subclass in the viewDidLoad() method.Also, in property definitions, don't feel like you've done initialization by writing the default values.
Next, take a look at each reference that you can view to help with Xcode and make sure you have a better choice than the methods you're about to use.
Please refer to the code below.

import UIKit
import AVFoundation

classViewController:UIViewController, AVAudioPlayerDelegate{

    @IBOutlet weak varplayButton:UIButton!
    @IBOutlet weak var volumeSlider:UISlider!

    varcymbalPlayer —AVAudioPlayer?

    override func viewDidLoad(){
        super.viewDidLoad()

        iflet url=Bundle.main.url(forResource: "cymbal", withExtension: "mp3"){// If the resource is not complex with tiering, resource files can be directly designated.
            cymbalPlayer=try?AVAudioPlayer(contentsOf:url)//Use simple exception handling patterns.
            cymbalPlayer ?.delegate=self
            iflet value = cymbalPlayer ?.volume {
                volumeSlider.value = value
            }
        }
    }

    @ IBAction funcplayCymbal(_sender:UIButton){
        cymbalPlayer?.play()//If the initialization process is done properly, you can play it in just one line.
        sender.isEnabled=false//Do not press the button during playback.
    }

    // When the cymbalPlayerdelegate playback is finished, return the button to press.
    funcaudioPlayerDidFinishPlaying(_player:AVAudioPlayer, successfully flag:Bool){
        playButton.isEnabled=true
    }

    // volume control
    @ IBAction func volumeLebel(_sender:UISlider){
        cymbalPlayer?.volume=sender.value
    }
}

The AVAudioPlayer sound source has an initialization method with exception handling (throws) because it is a bit unreliable, for example, data downloaded on the network or files that can be referenced by music apps. The try? syntax replaces exception handling with Optional Chaining and Optional Binding.