I have updated the firmware and menu to the latest available and everything is working great, however sound is sometimes still missing until I power cycle. For example, I flashed three games and after finishing the system rebooted to the first game with sound. If I moved to the second of third game slot however, no sound. Move back to the first game and it had sound!
But, sometimes that does not happen and other weird things happen. For example, flashed some games yesterday and one was Nam '75. The game launched with sound, but just half sound. I was waiting for the guy to start talking the dialog, but it was not there...music was however. In game the music was there but the sound effects were not. After a power cycle the game worked fine.
I did notice when I first received the cart that having the menu music active would cause all sorts of strange behavior sound wise when the game(s) were launched, so I shut that off. Since then the issues are not as bad, but they are there still. A power cycle in my experience always fixes the problem with said game.