Are you using an audio receiver? How are you connecting your PS3 to your TV/Receiver?
I would say make sure all your connections are secure, check to make sure your HDMI cable isnt bent or anything. If you using optical make sure the cable is in all the way. Then last (if your using a audio receiver) make sure your settings are all correct according with your PS3.
I have a audio receiver that wont make sound until it detects sound, and when there is no sound going in, it sorta goes on a standby. For instance, when I had the menu "clicks" on my PS3, I would only hear about half of them because of my receiver was going on and off. Maybe you have a similar problem?