Audio {
        source: "file:///home/amir/Downloads/beautiful.mp3"
        playbackRate: 0.5
        onStatusChanged: play()
    }

Mine isn't working. I tried different values on playbackRate but it plays normal speed. Xubuntu18.04 / Qt 5.12.7

mp3 sounds that are in playlist and loaded from arbitrary folder

I put in a fixed url: file:///home/fcarney/Music/melting.mp3
It played just fine. I could adjust the playback rate. However, for all sounds you cannot adjust rate while it is playing. It only seems to accept rate when it starts playing back.

You will need to provide your own sound. Mine is in a qrc file with "sound" prefix.

import QtQuick 2.12
import QtQuick.Window 2.12
import QtMultimedia 5.12
import QtQuick.Controls 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Audio Test")

    Button {
        id: button

        text: "play sound"
        onClicked: {
            console.log(text)
            audio.play()
        }
    }
    Slider {
        id: slider

        anchors.top: button.bottom

        from: 0.1
        to: 2.0
        value: 1.0

        width: 200
    }

    Audio {
        id: audio

        playbackRate: slider.value

        source: "sound/haha.ogg"
    }
}