usb to roku 2 to tv to surround speakers

Hey, I'd like to just make sure it's possible before I spend more time trying to get this to work, and maybe get some friendly hints towards really obvious things that I might be doing wrong.

I have movie files on a usb drive. (.avi files)
hdmi cable connects the roku 2 xs to our sony bravia tv.
audio out from the tv connects to our onkyo audio hub.
the video plays fine, but a notification pops up that says no compatible audio file was detected.

we are able to use the roku to watch netflix fine, and the sound works through the surround sound speakers connected to the onkyo.
also, i've connected a laptop to the tv using hdmi, but I only remember getting sound to play through the tv speakers; we didn't have the audio out from the tv connected to the onkyo to try that.

any advice on what to mess around with would be awesome thanks.