1. In the PS3's XMB, go to the left and select Settings.
2. In Settings, go into Sound Settings.
3. Select Audio Output Settings, and select the type of cable you use.
4. The next page will give you an option for Automatic or Manual. Select Manual.
5. The next page will give you a list of all the audio settings the PS3 can output. Unselect every option except the ones grayed out, which are required.
6. The next page will ask you to confirm the selections, say yes.
7. (OPTIONAL) For those using HDMI cables, now that we have set them to default settings, we can actually start the process over again, but instead of selecting Manual we will select Automatic. This will force the PS3 to scan the devices again and set up the correct list of output modes.
If the sound plays correctly now in the game, then you may need to review your television or speaker settings / configuration before trying to enable surround sound again.