sound help for ps3

[moocow]

k so i recently purchased a new ps3 cause my other one was stolen a while back and i set up everything exactly how i had my other one. but for some reason i cant get the sound to work right. i have a surround sound system its nothing great just one of those cheap ones from walmart. if i put it on the dts or dolby on the ps3 sound settings then it works fine for movies but for game demos sometimes the sound works and sometimes it wont it depends on the game. like for example ill play the bourne game demo and it wont have any sound when the dolby setting is turned on but if i have the dts one on its fine. then if i play a different game then i may not work with the dts or dolby settings turned on but like i said it works fine with movies no problem whatsoever