ALSA

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ALSA is the kernel-level sound API for Linux. On modern systems, it is usually used via a sound server like PulseAudio.

Troubleshooting ALSA

on a console fire up alsamixer

 alsamixer

you see plenty of vertical bars?
- you should be okay

you see very few vertical bars and the sound card (top-left) is something like "PC Speaker"?
- hit the 'S' key, you should be able to switch to the "real" audio card (if not your audio card is likely to not being supported yet).
- when the real audio card is selected you should be viewing the "plenty vertical bars" thing.
  - first thing to do is to disable pc speaker (kernel module "snd-pcsp", see below.

Make your audio card the default ALSA card

Sometimes the pc-speaker is the default audio card for ALSA. You can make your real sound card default instead. For example, if your sound card is "hda-intel" then add

boot.extraModprobeConfig = ''
  options snd slots=snd-hda-intel
'';

to your /etc/nixos/configuration.nix.

Sometimes, we may want to disable one of intel cards. Here is how to disable first card, but enable the second one.

boot.extraModprobeConfig = ''
  options snd_hda_intel enable=0,1
'';

Alternatively you can ...

Disable PC Speaker "audio card"

edit /etc/nixos/configuration.nix and add "snd_pcsp" to boot.blacklistedKernelModules option:

boot.blacklistedKernelModules = [ "snd_pcsp" ];

Now reboot and retry from the beginning (i.e. check that your real card is shown by alsamixer without using the 'S' key).

Other hardware specific problems

Some hardware specific problems can be resolved by adjusting the options for the sound module. For example, the microphone may be stuck on an unusably low volume. First you should be sure that you have already checked the settings in alsamixer to make sure nothing is muted, and also any physical buttons on your computer (I have twice overlooked the mute button on laptops!).

You should be able to look up the available options for model in models.rst. You can try them out interactively as follows:

Close any applications using the sound card
1. See if any applications are using the sound card
  $ lsof /dev/snd/*
  
  COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
  
  pulseaudi 14080 goibhniu 30u CHR 116,7 0t0 5169 /dev/snd/controlC0
  
  pulseaudi 14080 goibhniu 37u CHR 116,7 0t0 5169 /dev/snd/controlC0
2. Kill them
  for any process apart from pulseaudio you could just do:
  
  $ kill -9 14080
  
  but in the case of pulseaudio you have to prevent it from respawning itself automatically
  
  $ mkdir -P ~/.config/pulse && echo "autospawn=no" >> ~/.config/pulse/client.conf
  
  you can then stop pulseaudio with:
  
  $ pulseaudio -k # or kill it by process id
Unload the snd-hda-intel module
rmmod snd-hda-intel
Find your model
grep Codec /proc/asound/card0/codec*
Look up the model options for your card
Try each one
modprobe snd-hda-intel model=3stack-6ch
Test if this has fixed your problem (tip: aplay and arecord are alsa based command line tools you can use to quickly check)
Repeat until you have exhausted all the options or have fixed your problem
TIDY UP!
Don't forget to re-enable pulse autospawning by changing "autospawn=no" to "autospawn=yes" in ~/.config/pulse/client.conf

Once you have found a setting that works, you can add it to your configuration file:

boot.extraModprobeConfig = ''
  options snd-hda-intel model=YOUR_MODEL 
'';

Much of this is taken from https://help.ubuntu.com/community/HdaIntelSoundHowto which also has additional tips.