Without makes and models of the Roku, TV and surround sound we can't be exact.
Your options are to connect the Roku to the TV with HDMI. If your TV and surround sound both have ARC make sure that it is turned on and that the surround sound is connected to the HDMI ARC input of the TV.
If you don't have ARC then the audio out of the TV to the surround sound should work.
Last option is to connect the Roku audio out directly the the surround sound. If it doesn't have audio out then an HDMI audio extractor will work.