If the sound file isn't found it is most likely that you've entered the path incorrectly. I recommend using the Browse button to locate the file(s).
To play a random sound at bust out, create the various Sounds you'd like to use, then create a Sound Event for "when a player busts out" for each of the sound files (if you have 6 sounds, create 6 sound events). After you create the 2nd one, the software will warn you that you've created more than one sound event for the same event, and one will be chosen at random when the event occurs. And so on for each sound.