<property>
Defins which property supplies the value for the calculation.
+ Either a <property> or an <internal> should be defined.
The value is treatened as a floating point number.
+
+ <internal>
+ Defins which internal variable should be used for the calculation.
+ The value is treatened as a floating point number.
+ The following internals are available at this time:
+
+ dt_play: the number of seconds since the sound started playing.
+
+ dt_stop: the number of seconds after the sound has stopped.
<type>
Defines the function that should be used upon the property