PulseAudio
PulseAudio is a popular sound server for Linux. It is now required by a number of applications, and should be enabled if audio support is desired on NixOS. Enabling PulseAudio is sufficient to enable audio support on NixOS in most cases.
Enabling PulseAudio
Add to your configuration:
hardware.pulseaudio.enable = true;
hardware.pulseaudio.support32Bit = true; ## If compatibility with 32-bit applications is desired.
You may need to add users to the audio group for them to be able to use audio devices:
users.extraUsers.alice.extraGroups = [ "audio" ... ];
Troubleshooting PulseAudio
Before troubleshooting PulseAudio, determine that the kernel-level sound APIs (ALSA) are functional; see ALSA.
If ALSA-level audio is working, determine whether audio is being routed via PulseAudio.
To determine what processes are using the sound devices:
$ sudo 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
In this example, only pulseaudio processes are using sound devices.
If other processes (such as plugin-container) are using sound devices, this indicates they are bypassing PulseAudio; check that you don't have a local ~/.asoundrc file directing audio to somewhere else.
Note that you may need to enable the full pulseaudio package using:
hardware.pulseaudio.package = pkgs.pulseaudioFull;
For example I had to enable this package in order to solve an error:
snd_pcm_open failed: Device or resource busy
(The problem is that also also tries to connect to the card that is already used by pulseaudio, so we need a module on pulseaudio to redirect also calls to pulseaudio)
Explicit PulseAudio support in applications
Normally, the system-wide ALSA configuration (/etc/asound.conf) redirects the audio of applications which use the ALSA API through PulseAudio. For this reason, most applications do not need to be PulseAudio-aware. Some NixOS packages can be built with explicit PulseAudio support which is disabled by default. This support can be enabled in all applicable packages by setting:
nixpkgs.config.pulseaudio = true;
Using Pulseaudio Equalizer
Currently (2017-11-29 
#8384) the qpaeq command does not work out of the box, use the following commands to get it running:
$ pactl load-module module-equalizer-sink
$ pactl load-module module-dbus-protocol
$ nix-shell -p python27Full python27Packages.pyqt4 python27Packages.dbus-python --command qpaeq
Clicking and Garbled Audio for Creative Sound Cards
If you own a sound card from Creative like X-Fi and you notice random clicks during audio playback, then add following to your configuration [1]:
hardware.pulseaudio.configFile = pkgs.runCommand "default.pa" {} ''
sed 's/module-udev-detect$/module-udev-detect tsched=0/' \
${pkgs.pulseaudio}/etc/pulse/default.pa > $out
'';
Then perform # nixos-rebuild switch, followed by $ pulseaudio -k.
The difference should be directly noticeable. This is a known issue related to quality of Creative driver [2].