Just crack it open and either clip the leads to the "speaker" or put a few layers of tape over it. I too hate appliances with loud beeps - or the new annoying trend of playing songs (usually from the Asian manufacturers).

Love my Breville toaster oven though - thing is so stinking versatile and by far the best of any I have ever used. We use it as an oven far more than as a toaster. Perfect for heating/baking most stuff without heating up the entire house like the regular oven does. We have the one that also has the air fry mode - use that mode all the time to cook vegetables that are delicious and a lot healthier too. Did I mention we really love our Breville toaster oven? And to think I originally chose it because it was the only one I could find that had a stinking light inside?