Ok,
I read another reply from an earlier posting. It suggested that the tama got overheated by being left in the car or a bag or something and that effects the sound. I don't know if it is true because we live in a cold weather state and it has been freezing, but anyway, it said to just press the reset button on the back. An egg will show up. You press button B and then use a button to select download and then b button to execute. That just took us back to the last time he was sleeping and the sound is back to normal. If you choose reset, you will start completely over, like it is a brand new tamagotchi, so if you don't want to do that, then make sure you select download.
As to why the sound changed, I still don't know. Most people just explain it as a glitch. If you like the sounds, you can leave them I guess.