working audio in the Raspberry Pi 4 through the headphone jack

As it is said in the Debian wiki, headphone audio does not work out-of-the-box with the generated images. In my case, with a Raspberry Pi 4B 8GB, and the image rpi4-20210726, I managed to make it work, persistently across reboots. The procedure boils down to this:

# amixer -c 2 cset numid=2 on
# amixer -c 2 cset numid=3 1

The first one is to enable PCM, and the second one is to route audio to the jack (0=auto 1=jack 2=hdmi). This should be done, if possible, before attempting to output any sound, and before logging into the desktop. I think doing it as root is enough; if it isn't, it can be done as normal user too. Assuming it is the card "2". In my case it is, as seen in the output of aplay -l:

user@rpi4-20210726:~$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: vc4hdmi0 [vc4-hdmi-0], device 0: MAI PCM vc4-hdmi-hifi-0 [MAI PCM vc4-hdmi-hifi-0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: vc4hdmi1 [vc4-hdmi-1], device 0: MAI PCM vc4-hdmi-hifi-0 [MAI PCM vc4-hdmi-hifi-0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
  Subdevices: 6/7
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
card 2: ALSA [bcm2835 ALSA], device 1: bcm2835 IEC958/HDMI [bcm2835 IEC958/HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Card state can be printed with:

amixer  -c 2 cget numid=1
amixer  -c 2 cget numid=2
amixer  -c 2 cget numid=3
amixer  -c 2 cget numid=4
amixer  -c 2 cget numid=5

In my case, the output of the last commands is:

numid=1,iface=MIXER,name='PCM Playback Volume'
  ; type=INTEGER,access=rw---R--,values=1,min=-10239,max=400,step=0
  : values=-1158
  | dBscale-min=-102.39dB,step=0.01dB,mute=1
numid=2,iface=MIXER,name='PCM Playback Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=on
numid=3,iface=MIXER,name='PCM Playback Route'
  ; type=INTEGER,access=rw------,values=1,min=0,max=2,step=0
  : values=1
numid=4,iface=PCM,name='IEC958 Playback Default'
  ; type=IEC958,access=rw------,values=1
  : values=[AES0=0x00 AES1=0x00 AES2=0x00 AES3=0x00]
numid=5,iface=PCM,name='IEC958 Playback Con Mask'
  ; type=IEC958,access=r-------,values=1
  : values=[AES0=0x02 AES1=0x00 AES2=0x00 AES3=0x00]

The configuration can be tested with:

speaker-test -D sysdefault:CARD=2 -c 2 -l 1
aplay -D sysdefault:CARD=2 -r 48000 -f S16_LE -c 2 /dev/urandom

or simply:

speaker-test -c 2 -l 1
aplay -r 48000 -f S16_LE -c 2 /dev/urandom

The configuration is preserved across reboots, thanks to alsa-restore and /var/lib/alsa/asound.state. It can be reset to 'default' state doing:

# service alsa-restore stop
# rm /var/lib/alsa/asound.state
# shutdown -h now
(and power off)

If this configuration works for others it might be useful to put it in the wiki or integrate it into the images.

After testing this configuration, I got distorted sound. I solved it by looking for the card number again:

user@rpi4-20210726:~$ cat /proc/asound/cards 
 0 [vc4hdmi0       ]: vc4-hdmi - vc4-hdmi-0
                      vc4-hdmi-0
 1 [vc4hdmi1       ]: vc4-hdmi - vc4-hdmi-1
                      vc4-hdmi-1
 2 [ALSA           ]: bcm2835_alsa - bcm2835 ALSA
                      bcm2835 ALSA

and creating the file /etc/asound.conf with this text:

pcm.!default {
    type hw
    card 2
    device 0
}

ctl.!default {
    type hw
    card 2
    device 0
}