Many of those convertor boxes fail on apps until you find the app's settings for audio coding. You should try Stereo or PCM while in the app and the TV settings.
Again, APPs can have audio settings. There is NO STANDARD so I'm always fishing around the app settings.
I just bought a new Samsung 55" 4K ultra HD flat screen TV. I have an optical audio cable plugged into the TV that goes to a coverter box to convert to RCA conections which then plug into a home stereo reciever playing sound through my home speakers. The sound is perfect when watching anything from cable box but when I try to stream youtube through the TV via Wifi all I get is scratchy static for sound all the time. I changed the sound output option from the optical cord output to the TV speakers and it works fine, no static. Thank you for any help!