This module works pretty similar to the module-role-cork.
It should be used as an alternative to that module. Basically
it decreases the volume of the streams specified in ducking_roles
in the presence of at least one stream specified in trigger_roles.
Also, it's possible to choice the volume that will be used in the
ducking streams and if it should operates in all devices or not.
For basic reference: http://en.wikipedia.org/wiki/Ducking
