Ubuntu 22.04: no audio on the speakers through Line out

I am running Kubuntu 22.04.3 LTS with KDE Plasma 5.24.7. I would like to use the Line Out (Built-in Audio Analog Stereo) as audio output. These are the audio devices shown by lspci:

$ lspci -knn | grep -i audio
00:03.0 Audio device [0403]: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller [8086:0c0c] (rev 06)
    Subsystem: ASUSTeK Computer Inc. Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller [1043:8534]
00:1b.0 Audio device [0403]: Intel Corporation 8 Series/C220 Series Chipset High Definition Audio Controller [8086:8c20] (rev 05)
    Subsystem: ASUSTeK Computer Inc. 8 Series/C220 Series Chipset High Definition Audio Controller [1043:8573]

I connected the stereo speakers with a jack cable to the Line Out (Built-in Audio Analog Stereo); stereo headphones are also connected with a jack to the front-panel audio output of the case.

This way, the system detects that a jack is connected to Line Out and Line Out is automatically selected as default sound output line. Also, almost every time I click in the sound icon in the bottom right corner of the task bar (near the clock), I hear a spike in the audio speakers. The same happens if I run PulseAudio.

However, if I try to play anything (from YouTube, from VLC, ...), there's no audio coming out from the speakers. If instead I select the headphones, they work as expected.

In PulseAudio Volume Control -> Output Devices -> Port, it is shown "Line Out (plugged in)" with almost 100 % volume. If I play something, the visual sound level even moves according to the audio, but there's nothing coming out from the speakers, as if "Mute" was active (but it isn't).

What could it be the problem?

If I physically unplug the headphones from the case front panel, the audio is correctly played in the speakers. This is annoying, but at least it works this way.

If there's something I can do to avoid unplugging the headphones each time, any suggestion is welcome.