I have now found a reproducible workaround for the ASUS ExpertBook P3
PM3606CHA (ALC256, subsystem 1043:3501).


The existing alc256-asus-headset model fixes jack detection and automatic 
Speaker/Headphone switching, but audio playback remains silent without 
additional codec initialization.


I compared Linux codec state against Realtek Windows driver 6.0.9932.1 using 
RtHDDump, including dumps captured while audio was actively playing.


A working Linux initialization has been reproduced using sysfs init_verbs + 
reconfig:

```
NID 0x20:
coefficient 0x10 = 0x7f20
coefficient 0x16 = 0x0c50
coefficient 0x35 = 0x8d6a
coefficient 0x37 = 0xfe06
coefficient 0x57 = 0x7f7f
```

GPIO2 is then configured:

```
SET_GPIO_MASK      0x04
SET_GPIO_DIRECTION 0x04
SET_GPIO_DATA      0x04
```

With this initialization plus alc256-asus-headset:


Internal speakers work
3.5 mm headphones work
Jack detection works
Automatic Speaker ↔ Headphone switching works
Switching continues to work while an audio stream is actively playing
Repeated plug/unplug cycles work correctly
The result survives cold boot when the initialization is applied before the 
user PipeWire session starts

One interesting runtime observation: immediately after codec
reconfiguration, coefficient 0x16 may read 0x2c50; when playback starts
it changes to 0x0c50, matching the working Windows ACTIVE state.

I am currently using a systemd oneshot service before the display
manager to apply the initialization. This has been verified after a
clean reboot.

I have Windows speaker/headphone IDLE and ACTIVE RtHDDump files and
corresponding Linux codec dumps available if useful for an upstream
quirk.

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https://bugs.launchpad.net/bugs/2167288

Title:
  No internal speaker or 3.5mm headphone audio on ASUS ExpertBook
  PM3606CHA - Realtek ALC256 subsystem 1043:3501

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