My environment is Ubuntu 24.04.5 LTS, and I can confirm that the issue does not occur with Qt 6.8.3 either. (The application ran for more than five minutes without freezing.)

In my environment, sound also works with Qt 6.12.0, but the application eventually freezes and produces a large number of error messages.

The FFmpeg versions used by Qt Multimedia are the same in my environment as well:

• Qt 6.8.3
  • Using Qt multimedia with FFmpeg version 7.1 LGPL version 2.1 or later
• Qt 6.12.0
  • Using Qt multimedia with FFmpeg version 9.0.1 LGPL version 2.1 or later

It's interesting that different issues seem to occur depending on the Ubuntu version.

So (after a week's delay 😅) that is what I have done.

My OS is Ubuntu 22.04.5 LTS jammy

I generated the wav files as follows:

sox -n -r 48000 -b 16 tone_1.wav synth 0.6 sin 440
sox -n -r 48000 -b 16 tone_2.wav synth 0.5 sin 240

When I launched the code using Qt_desktop/6.8.3/gcc_64/bin/qmlscene, things seem to work pretty well. I cannot reproduce the reported issue.

I then tried with both:

• Qt_desktop/6.12.0/gcc_64/bin/qmlscene and also
• Qt_desktop/6.12.0/gcc_64/bin/qml

In both of these 6.12.0 attempts, I hear no sound whatsoever! 🫤

So I still cannot reproduce the reported issue, but I definitely see different outcomes between 6.8.3 and 6.12.0, which the behavior in 6.12.0 seeming to be worse. (But maybe there is some other dependency I am missing to use multimedia in 6.12.0 on Ubuntu 22.04? .... but there wasn't any warning or error printed that would hint in that direction...)

When I use 6.8.3 I see:

qt.multimedia.ffmpeg: Using Qt multimedia with FFmpeg version 7.1 LGPL version 2.1 or later

And with 6.12.0 I see:

qt.multimedia.ffmpeg: Using Qt multimedia with FFmpeg version 9.0.1 LGPL version 2.1 or later

It's a single QML file with two SoundEffect instances that do stop() → change source → play() on a timer,
the same way as in my game. No C++ or build step is needed.

Setup

soundeffect_repro/
  Main.qml
  sounds/tone_1.wav
  sounds/tone_2.wav

Sorry, the WAV files (tone_1.wav and tone_2.wav) can't be uploaded, so you'll need to prepare two short WAV files yourself.
Two different short WAV files are needed. Mine are 16 bit PCM, stereo, 44.1 kHz sine waves of 0.5 s and 0.6 s.

Run

<Qt>/6.12.0/gcc_64/bin/qml Main.qml 2> err.txt
grep -m3 -n QSocketNotifier err.txt

I ran it 5 times, and it reproduced every time, after about 3 to 30 s.

13:QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread
14:QSocketNotifier: Invalid socket 60 and type 'Read', disabling...
15:QSocketNotifier: Invalid socket 60 and type 'Read', disabling...

When it reproduces:

• err.txt fills up with QSocketNotifier: Invalid socket ... lines.
• The square in the window stops rotating, the play count stops increasing, and no more sound is played
  (see the screenshot below).

Note: once the issue starts, err.txt keeps growing by tens of MB per second.
Please stop the app (Ctrl+C) soon after it reproduces, or the file will get huge.

Main.qml

pragma ComponentBehavior: Bound
import QtQuick
import QtMultimedia

Window {
    id: root

    readonly property int playInterval: 800    // ms between plays of each SoundEffect
    readonly property list<url> soundUrls: [
        Qt.resolvedUrl("sounds/tone_1.wav"),
        Qt.resolvedUrl("sounds/tone_2.wav")
    ]
    property int playCount: 0

    width: 360
    height: 120
    visible: true
    title: "SoundEffect repro"

    // Two SoundEffect instances, the second one starting half an interval later
    Repeater {
        model: 2

        delegate: Item {
            id: player

            required property int index
            property int nextSound: index   // index into soundUrls

            function playNext() {
                effect.stop()
                effect.source = root.soundUrls[player.nextSound]   // a different file from the previous play
                effect.play()                                      // without waiting for SoundEffect.Ready
                player.nextSound = (player.nextSound + 1) % root.soundUrls.length
                root.playCount++
                console.log("play " + root.playCount + " instance " + player.index + " "
                            + effect.source.toString().split("/").pop())
            }

            SoundEffect {
                id: effect

                onStatusChanged: {
                    if (status === SoundEffect.Error)
                        console.log("instance " + player.index + ": Error " + source)
                }
            }

            Timer {
                interval: player.index * root.playInterval / 2
                running: true
                onTriggered: {
                    player.playNext()
                    repeatTimer.start()
                }
            }

            Timer {
                id: repeatTimer
                interval: root.playInterval
                repeat: true
                onTriggered: player.playNext()
            }
        }
    }

    // Progress line every 10 s; it stops once the issue starts
    Timer {
        interval: 10000
        running: true
        repeat: true
        onTriggered: console.log("progress: plays=" + root.playCount)
    }

    // Shows whether the GUI thread is still alive
    Rectangle {
        x: 20
        y: 30
        width: 60
        height: 60
        color: "steelblue"
        RotationAnimation on rotation {
            from: 0
            to: 360
            duration: 2000
            loops: Animation.Infinite
        }
    }

    Text {
        x: 110
        y: 50
        text: "plays: " + root.playCount
    }
}

Thanks again!

Sharing that small project would be nice as it will allow other people to try to reproduce your issue.