Is it plugged into a receiver? Look into preferences of the PS3. If the DVD is 48khz and the game is 96khz and your receiver or TV only supports certain khz then thats probably what it is.
There is an option in there for 44.1 or 44.1/88.2/176.4, assuming the single 44.1 is more stable. Try switching the option there under audio properties and see what happens