The receiver doesn't support Dolby Digital+. I have the 510 on a secondary system, so I have to set the audio to PCM when using streaming boxes that use DD+.
This is good advice for most apps, but if the OP uses Netflix or Amazon Prime, the Roku cannot convert the DD+ to regular DD, so there won't be any sound. If those apps are used, PCM is the only format that will work.Easy fix if the Roku Express is like the other Roku devices.
Go into Settings
change from Auto Detect to either Dolby Digital or Dolby Digital,DTS
When set to Auto Detect it will send Dolby Digital Plus which your Sony can't decode. So change it to Dolby Digital or Dolby Digital, DTS
Don't change to PCM. You want something more than 2 channel stereo if possible.