mixerctl
From NetBSD Wiki
With mixerctl(1) you can change the in-kernel audio mixer settings. The available settings depend on the audio driver.
The following command requests the current master output volume:
$ mixerctl outputs.master outputs.master=127,127
To increase the output volume, we can do something like:
$ mixerctl -w outputs.master=150,150 outputs.master: 127,127 -> 151,151
This is not exactly the same as the chosen value because the output can assume only a limited number of values in the range 0,255. The exact possible values can be requested using the -v switch:
$ mixerctl -v outputs.master outputs.master=151,151 volume delta=8
This indicates that the value is a volume setting and can be modified in multiples of 8.
If you wish to make the settings persistent across reboots, you can put the assignment in /etc/mixerctl.conf, which will be read upon boot:
outputs.master=151,151
