stumped on this audio question

[un4gvn94538]

this was asked in another site. i cannot figure the answer to this. i have a ps3 and an onkyo 705. i can matrix the sound from dvd and blu to 7.1 but i cannot matrix the sound from movies streaming from my computer or playing on the ps3 hard drive. even if the video has dd 5.1 encoded in it, i cannot matrix it to 7.1. i also tried a ps3 game and was unable to matrix it to 7.1.
i dont think it matters wether im bitstreaming or using lpcm because either way with dvd i can matrix the sound.

can anyone fill me in on this?
thanks

added: also on 2channel divix sources i can select an option on my receiver for 5.1 matrix but the sound will not come out. i just get a faint hiss out of the rear speakers.