An ISoundEventDefinition object represents a sound event in the game. A sound event is triggered when a sound is about to be played.

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import mods.contenttweaker.SoundEvent;

You can get such an object using the Sound Event Bracket Handler:
<soundevent:ambient.cave>

ZenMethod	Return type	Definition
ZenMethod getSoundName()	Return type String	Definition Returns the event's sound's name