Sound from built-in laptop speaker iff plugging in headphones

[tkpapp]

I am running Ubuntu 25.04, installed as KUbuntu but running a Sway desktop, on a Lenovo IDEAPAD 5-14ITL05 laptop.

My problem is that

1. if I do not plug in a speaker/headphones to the 3.5mm audio port, I have no sound from internal speakers, they are greyed out in pavucontrol
2. if I plug in a speaker/headphones to the above port, I get sound from both the latter and the build-in laptop speakers.

From lspci, I have an

00:1f.3 Multimedia audio controller: Intel Corporation Tiger Lake-LP Smart Sound Technology Audio Controller (rev 20)

Systemd appears to start everything correctly:

$ pstree | grep pipe
        |         |-pipewire---2*[{pipewire}]
        |         |-pipewire---{pipewire}
        |         |-pipewire-pulse---2*[{pipewire-pulse}]

I attach the output of pactl sinks both plugged an unplugged, their diff is

$ diff /tmp/pactl.sinks.plugged /tmp/pactl.sinks.unplugged 
289c289
<       Volume: front-left: 26202 /  40% / -23.89 dB,   front-right: 26202 /  40% / -23.89 dB
---
>       Volume: front-left: 65536 / 100% / 0.00 dB,   front-right: 65536 / 100% / 0.00 dB
294c294
<       Flags: HARDWARE HW_MUTE_CTRL HW_VOLUME_CTRL DECIBEL_VOLUME LATENCY 
---
>       Flags: HARDWARE DECIBEL_VOLUME LATENCY 
368c368
<               [Out] Headphones: Headphones (type: Headphones, priority: 200, availability group: Headphone, available)
---
>               [Out] Headphones: Headphones (type: Headphones, priority: 200, availability group: Headphone, not available)

Kernel is 6.14.0-24-generic.

Any help would be appreciated, let me know if I can provide more information.

[tkpapp]

(@rubi1200: thanks for the well-meaning title correction, iff = if and only if is intentional in the title)

[tkpapp]

I figured out by experimenting that the phenomenon described above only happens when the HDMI cable (connecting to a display) is plugged in. If the HDMI cable is unplugged, the laptop speakers work fine and sound output auto-switches to the audio jack if that is plugged in.

pavucontrol screenshot with HDMI cable unplugged:

pavucontrol screenshot with HDMI cable plugged in:

[rubi1200]

Are you able to reproduce this on other versions, 24.04 for example?