Play sound on button click in PyQt6

I am trying to create 2 buttons that would play a sound once and endlessly. But at this stage it is not possible to reproduce the sound at all. What am I doing wrong?

import sys
from PyQt6.QtCore import QUrl
from PyQt6.QtMultimedia import QMediaPlayer, QAudioOutput
from PyQt6.QtWidgets import QWidget, QPushButton, QVBoxLayout, QApplication


class SoundPlayer(QWidget):
    def __init__(self):
        super().__init__()

        filename = "sound.mp3"
        self.player = QMediaPlayer()
        self.audio_output = QAudioOutput()
        self.player.setAudioOutput(self.audio_output)
        self.player.setSource(QUrl.fromLocalFile(filename))
        self.audio_output.setVolume(50)
        # player.play()

        self.play_button = QPushButton('Play')
        self.play_endlessly_button = QPushButton('Play Endlessly')

        layout = QVBoxLayout()
        layout.addWidget(self.play_button)
        layout.addWidget(self.play_endlessly_button)
        self.setLayout(layout)

        self.play_button.clicked.connect(self.play_once)
        self.play_endlessly_button.clicked.connect(self.play_endlessly)

    def play_once(self):
        self.player.play()

    def play_endlessly(self):
        self.player.setLoops(QMediaPlayer.Loops.Infinite)
        self.player.play()

if __name__ == '__main__':
    app = QApplication([])
    widget = SoundPlayer()
    widget.show()
    sys.exit(app.exec())

Tried using the "play()" method of the QMediaPlayer class.
It is necessary that when the buttons are pressed, the sound is played once and endlessly according to the buttons.