Easy question

Hello
Newbie here.

I have connected my 5.1 creative speakers directly to my HTPC with XBMC.

The settings are ok (direct sound - speakers/analog)

I played some DTS and AC3 movies and the sound is great.

My question is: what i am hearing is really AC3/DTS? I can hear sounds going from front to rear with great definition.

Does XBMC have a inbuilt decoder?

Thanks!