Mediastar 8200s Sound Problem

[RedXT]

When playing movies (avi,mp4,mkv) Connected using optical, on the medistar I get dolby surround sound through my amp if the sound track is AC3 6 CH but if it is AAC 6CH all I get is PCM48. I have the same problem if I use the Noontec A3S, but through the usb input on my samsung AC3 and AAC all play full surround

Any Ideas anyone