asoundrc

The .asoundrc file allows a user to leverage extra functionality offered by ALSA. This article is supplemental to the information available on the ALSA wiki

The .asoundrc file uses a simple syntax that consists of code blocks enclosed by curly braces to create so-called definitions. Definitions can be created for several types of devices. The most used are:

1. pcm devices. These can be actual hardware or virtual devices created in the .asoundrc file itself.
2. pcm_slave devices. These are secondary references to either hardware or virtual devices. They can be used to chain definitions.
3. ctl devices. These comprise how programs can modify specific parameters of hardware or virtual devices.

To assign a definition to a device add a line beginning with one of the key words above followed by a dot and the name of the device. This name can be a reference to the default card (!default) or a designation of choice.

Definitions must contain at least the type key word followed by the name of the PCM plugin that has to be configured. A full list of available plugins can be found here: https://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html

Asoundrc
ALSA project - the C library reference
A close look at ALSA