Yes, the Roku player can support pass-through of 5.1 surround sound to your TV or AV receiver.
To enable surround mode on Roku, go to Settings>Audio>Audio mode and select "Auto".
Note: 5.1 surround sound requires a receiver capable of decoding Dolby Digital and/or Dolby Digital Plus, depending on the audio format in the content.
Here are some services on Roku that support Dolby Digital or Dolby Digital Plus:
Amazon Instant Video
FandangoNOW Movies and TV
Programs that offer 5.1 surround sound can be identified by a "5.1" icon displayed near the parental guidance ratings on movie and TV show detail pages in the Netflix and Amazon channels. We expect other partners to add support for 5.1 surround sound in the future.