pamixer

TLDR

List all sinks and sources with their corresponding IDs

$ pamixer --list-sinks --list-sources

Set the volume to 75% on the default sink

$ pamixer --set-volume [75]

Toggle mute on a sink other than the default

$ pamixer --toggle-mute --sink [ID]

Increase the volume on default sink by 5%

$ pamixer --increase [5]

Decrease the volume on a source by 5%

$ pamixer --decrease [5] --source [ID]

Use the allow boost option to increase, decrease, or set the volume above 100%

$ pamixer --set-volume [105] --allow-boost

Mute the default sink (use --unmute instead to unmute)

$ pamixer --mute