vol <args>
--help show this message
--show show system audio status
--token=<id> select the device by token
--uid=<uid> select the device by partial UID
--input select the default input device
--gain=<db> set system audio gain
--mute=(on|off) mute/unmute system audio
--agc=(on|off) enable/disable AGC
Given no arguments, vol waits for the following keystrokes
+ increase system gain
- decrease system gain
m toggle mute
a toggle agc
enter quit