When your sound stops have you tried to see if it is working with headphones or Bluetooth? If there is no sound using headphones or a Bluetooth device there's probably an issue with the phone itself. If you've done a stock Odin flash and its still doing it, I would have to guess the phone is bad. Send it back. The only problem you might have is if they check the flash count, you may end up paying for a new phone. I've never had to use Triangle Away to reset my flash counter, but I believe it needs to be flashed in a custom recovery (which you don't have).
Sent from my SCH-I545 using Tapatalk 4