The "volume" command should set only the user's
personal volume preference, which should be persisted
The actual output volume should be the median of all
logged in user's volume preferences. If nobody is
logged in, the last median volume should be used. If
nobody has ever been logged in, the actual volume
should be set to some default value.
Using the median is less prone to abuse than the
average; if the average was used everybody would just
try to set the volume to 0 or 100 to get closer to what
they really want.