- The mixer_hwmute() function can be called when a soundcard receives a
mute request.
- The mixer_hwstep() function can be used to adjust the volume of one or
both channels.
- The 'hw.snd.hwvol_step' sysctl determines the amount that mixer_hwstep()
adjusts the volume by on each call.
- The 'hw.snd.hwvol_mixer' sysctl specifies the mixer device to adjust the
volume on for both functions. The values used correspond to the
SOUNDCARD_MIXER_* constants.
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