TrimUI Brick Pro (tg4040): volume keys don't control USB audio (USB-C headphones)

[bindike]

Device

TrimUI Brick Pro (tg4040), nightly v4.3.6-20260906

Summary

USB-C UAC headphones (Apple EarPods) play fine, but the volume keys and audio
routing aren't USB-audio-aware.

What works

• The headphones enumerate as ALSA card 1 (USB-Audio); the audio interfaces bind
  (snd-usb-audio is built in, CONFIG_SND_USB_AUDIO=y).
• If connected before launching a game, RetroArch (audio_device = "") opens the
  USB card and game audio plays through it (card1 pcm RUNNING, card0 closed).

Problems

1. Volume keys don't affect USB output. Volume on the Brick is handled by the
   stock TrimUI firmware via the transient /tmp/system/set_volume, targeting only
   the built-in codec (card 0). With audio on card 1, the keys (and the EarPods'
   inline HID volume buttons) do nothing audible. The USB card does expose volume
   (amixer -c1: numid=4 Headset Playback Volume 0-38, numid=6 0-120 — setting
   these works), but nothing drives it. There's no stable current-level value to
   mirror externally (codec controls sit at max; level is applied internally).
2. No hotplug re-routing. Output only lands on the USB card if it's present
   before the game starts; plugging in mid-game keeps output on the speaker until
   the game is relaunched.

Suggestion

Make the volume path and routing USB-audio-aware: when a USB audio card is present,
route output to it and have the volume keys target its playback volume
(card1 numid=4/6), ideally re-routing on hotplug.

[ryanmsartor]

Not considering this a high priority since the device has a 3.5mm jack, but we'll add it to the list. Thanks!