The DefineButtonSound can be used to emit a sound when an event occur on the specified button. It is likely better to use sprites that you display using actions than to use this tag. You will have access to more events and conditions, plus this tag always includes four sound effect references.
The f_button_id_ref is a reference to the button given sound effects.
There are four f_button_sound_condition. Each have a reference to a sound and some information on how to play it. The four conditions are given in the enumeration preceeding the DefineButtonSound structure.