[BUG] ASUS TUF A14 FA401EA: RT721-SDCA SoundWire speakers enumerate and stream but remain silent

Hi,

I am reporting an audio issue on an ASUS TUF Gaming A14 FA401EA using
the AMD ACP70 / RT721-SDCA SoundWire audio path.

Hardware / OS
- ASUS TUF Gaming A14 FA401EA
- AMD Ryzen AI platform / ACP70 SoundWire
- RT721-SDCA acting as jack + internal SmartAmp + DMIC
- Arch Linux / Omarchy
- Windows on the same machine plays through the internal speakers normally

Kernel work tested
- Stock Linux 7.1.9 did not expose the internal audio path correctly.
- A custom 7.1.9-arch1-2-fa401audio build using the existing FA401EA
SoundWire quirk lineage made ALSA/PipeWire enumerate the device
correctly, but produced no audible output.
- A follow-up 7.1.9-arch1-3-fa401audio build tested an
FA401EA-specific non-aggregated endpoint mapping. It still produced no
audible output.

Current ALSA enumeration
- card 1: amdsoundwire
- device 0: SDW1-PIN0-PLAYBACK-SimpleJack rt721-sdca-aif1-0
- device 2: SDW1-PIN1-PLAYBACK-SmartAmp rt721-sdca-aif2-2

PipeWire creates both:
- Audio Coprocessor Speaker
- Audio Coprocessor Headphones

The Speaker sink maps to hw:amdsoundwire,2 and can be selected as the
default sink.

Raw ALSA test
speaker-test -D hw:amdsoundwire,2 -c 2

The PCM opens successfully at 48 kHz / S16_LE / 2 channels, runs
continuously with no xruns or errors, and alternates Front Left /
Front Right, but there is total silence from the internal speakers.

Diagnostics performed during active playback
- PCM/DMA state: RUNNING
- RT721 SoundWire slave/runtime state: active
- ACP SoundWire manager: active
- DP3 SmartAmp sink port: active bank, both stereo channels enabled
- DP3 word length: 16-bit, matching the stream
- DAPM path: DP3RX -> FU21/FU23 -> SPK -> Speaker, with the relevant
widgets powered
- Speaker mixer controls: unmuted / full playback level
- RT721 driver power/mute sequencing: verified against the exact
driver source and appears to execute as intended
- CS31 sample-rate programming: 48 kHz path traced in the driver and
live state checked
- Function-unit input gain on entity 0x23 / control 0x0B: live
readback is 0xFE80 on both channels, matching the ACPI
mipi-sdca-function-initialization-table (approximately -1.5 dB), so
this is not a muted/default-gain issue
- ACPI DSDT/SSDT inspection found no separate GPIO/_DSM/WMI/EC
speaker-enable sequence outside the RT721 SDCA path
- Windows on the same hardware confirms the speakers/amp are
physically functional

Hypotheses already tested and ruled out
- PipeWire routing/default-sink issue
- ALSA mute/volume issue
- missing DAPM route or unpowered speaker widget
- incorrect sample rate
- missing ACPI speaker-enable GPIO/DSM/WMI method
- missing FU input-gain initialization
- FA401EA endpoint aggregation mismatch (tested in a dedicated
rebuild; did not restore output)
- a meaningful behavioural difference in the shared RT721 vs
RT722/RT712 speaker-init path was not found

At this point every Linux-visible layer from PCM through SoundWire
transport and SDCA control-plane state looks consistent during
playback, yet the speakers remain silent. The remaining unverified
area appears to be RT721 vendor-specific analog/index-register
initialization or another board-specific condition not visible through
the standardized SDCA controls.

A detailed local investigation report was generated, including the
live register/DAPM/ACPI findings and the failed endpoint experiment. I
can provide the full report, kernel patch, logs, ACPI tables, or any
targeted register/debug output that would be useful.

Please let me know what additional traces would be most helpful,
particularly around RT721 vendor analog/output initialization on a
self-contained SmartAmp topology.

Thanks,
James