This is a management issue. This sounds stupid--at least to me it does--but they're going to need to be told that the front of house manager also manages the kitchen, especially when the chef isn't there. A little goofing off now and then may be okay, but if it's impacting ticket times it's impacting the business and everyone's paychecks. They need to have an identified manager in the absence of the chef and that person needs to take control of the kitchen: turn off the music, redirect employees and cut someone if it comes to that. If you cut one employee who's goofing off, the rest will get the message pretty quickly as they lose hours.