mixerctl.conf —
audio mixer configuration file
The /etc/mixerctl.conf file consists of
  mixerctl(1) variables to set
  at boot time. Each line of mixerctl.conf has the
  following format:
variable=value
To generate a mixerctl.conf from the
    current mixer settings, execute:
mixerctl -a >
  /etc/mixerctl.conf
Set mixerctl to YES in
    rc.conf(5) to have the
    variables set at boot time. Additionally, you can have the settings saved
    and restored for the devices of your choice by listing them in
    mixerctl_mixers in
    rc.conf(5).
Example mixer settings for an esa(4)
  audio adapter:
outputs.master=255,255
outputs.master.mute=off
outputs.mono=255
outputs.mono.mute=on
outputs.mono.source=mixerout
outputs.headphones=255,255
outputs.headphones.mute=off
outputs.tone=255,255
inputs.speaker=255
inputs.speaker.mute=off
inputs.phone=191
inputs.phone.mute=on
inputs.mic=191
inputs.mic.mute=on
inputs.mic.preamp=off
inputs.mic.source=mic0
inputs.line=191,191
inputs.line.mute=on
inputs.cd=191,191
inputs.cd.mute=on
inputs.video=255,255
inputs.video.mute=off
inputs.aux=255,255
inputs.aux.mute=off
inputs.dac=191,191
inputs.dac.mute=off
record.source=mic
record.volume=255,255
record.volume.mute=off
record.mic=0
record.mic.mute=off
outputs.loudness=off
outputs.spatial=off
outputs.spatial.center=0
outputs.spatial.depth=0
 
The mixerctl.conf configuration file first appeared in
  NetBSD 2.0.