[Bug] Audio applet mouse slider has no effect on headphone volume (HDA card with multiple sinks)

[grissom-rasp]

Summary

On an Intel HDA card (sof-hda-dsp) that exposes headphones + 3 HDMI/DP sinks under the same PipeWire device.id, the COSMIC audio applet volume mouse slider has no audible effect on the headphone output. The slider moves visually, but the actual audio level does not change.

Keyboard Fn volume keys work correctly — only the applet's mouse slider is affected.

Relationship to #1388

This appears to be the same root cause as #1388 (ActiveRoute handler picks the wrong node from device_ids when multiple sinks share a device). The difference is:

• Audio applet: Volume slider has no effect after first volume change on multi-HDMI card #1388: multiple HDMI sinks on an AMD GPU
• This issue: headphones + 3 HDMI sinks on an Intel HDA card (sof-hda-dsp)

Steps to reproduce

1. Use a laptop with an Intel HDA audio controller that exposes multiple sinks under one device:

   Sinks:
       48. ... HDMI / DisplayPort 3 Output [vol: 0.64]
       56. ... HDMI / DisplayPort 2 Output [vol: 0.97]
       57. ... HDMI / DisplayPort 1 Output [vol: 0.46]
   *   58. ... Headphones                  [vol: 1.00]
   

   All four sinks share PipeWire device 49 (Tiger Lake-LP Smart Sound Technology Audio Controller).
2. Play audio through headphones.
3. Open the COSMIC audio applet and drag the volume slider with the mouse.
4. Observed: Slider moves visually; actual headphone volume does not change.
5. Use Fn+VolumeDown/Up keyboard keys instead.
6. Observed: Audio volume changes as expected.

Discrimination tests

To isolate the broken layer, I ran three tests while audio was playing:

Layer	Command	Audible change?
ALSA direct	amixer -c 0 set 'Headphone' 5% / 100%	YES
PipeWire	wpctl set-volume 58 0.05 / 1.0	YES
COSMIC applet mouse slider	Drag slider min→max	NO
COSMIC Fn keys	Fn+VolumeUp/Down	YES

This confirms the break is specifically in the applet's slider → PipeWire path, not in the audio stack.

Probable root cause

Same as #1388: the ActiveRoute handler in subscriptions/sound/src/lib.rs iterates device_ids (hash-based order) and picks the first node matching the device — which on a multi-sink card may be an HDMI output rather than the active headphone sink. The Fn keys bypass this path (they go through cosmic-osd → wpctl @DEFAULT_AUDIO_SINK@), which is why they work.

Environment

• OS: CachyOS Linux (Arch-based), kernel 7.0.8-1-cachyos
• COSMIC: 1.0.13-1.1 (cosmic-applets, cosmic-session, cosmic-comp)
• PipeWire: 1.6.5, WirePlumber: 0.5.14
• Hardware: HP EliteBook 840 Aero G8 (SBKPF), Intel Tiger Lake-LP Smart Sound Technology Audio Controller
• Codec: Realtek ALC285 (subsystem 103c:884b), driver: sof-hda-dsp
• ALSA card: sof-hda-dsp (card 0), UCM profile: HiFi

Workaround

Using wpctl set-volume <sink-id> <level> or Fn keys works. The applet mouse slider remains non-functional for volume changes on this configuration.