There is now an easier way to change a Boolean parameter field to a type-able parameter field.
When editing the Sound, if you want to enter a variable in one of the non-typable fields, set the caps lock key, then double-click the Sound to see its parameter fields. All parameter fields (including special fields like check boxes or radio buttons) will appear as fields where you can enter the value by typing. You can use this to set the value of a parameter field to a green variable, such as ?PhasePreserving.