What speakers are you using now? TV? Receiver and satellites? Are those speakers still good? If you're using an amp and satellites, did you accidentally switch to a different audio input? Are you able to get sound out of any other PS3 feature, like playing a movie or even the clicks in the XMB? What type of audio cable are you using? Has the cable become worn and frayed? Check your audio settings on the PS3. Have you set what audio formats are ok to output? ( Dolby Digital, DTS, etc? ) Perhaps that setting got changed somehow.
All else fails, reflash the firmware.