What audio format will videos played from a hard drive be delivered in?

I have plenty of movies and tv series that I have transferred to an external hard drive. I intend to play them via usb2.0 connected to a bluray player which is connected to an av receiver via hdmi, which in turn is connected to my tv also by hdmi. I'm happy that the 3d and hd quality video will be streamed to my tv, but I'm curious about the audio, will it have been copied in the same format and thus leave my hard drive in DD/DTS 5:1, or will it be in a simpler format and be down to my av receiver to decipher and kick out surround sound?

A friend mentioned selecting bit streaming audio on my bluray player but nothing in the audio settings mentions this, would this ensure all my vids came out in surround sound if I found and selected it?

Thanks