There is a FAQ for the device, found here:
https://streamview.com/faqs/
One of the listings includes this:
No playback of Dolby Digital sound from the AV amplifier when playing a multimedia or streaming file via Sun Player or VLC Player when the box is connected via Optical Digital Audio output
When playing multimedia files through different multimedia player applications, such as Sun Player or VLC Player, the audio processing of our box may conflict with the audio processing of the AV amplifier.
Therefore, we recommend you make the following settings in the multimedia player applications and leave the audio processing of the file to the AV amplifier so that the audio signal can be processed unchanged by our box from the AV amplifier. Sun Player or VLC Player:
If you have problems with the Dolby Digital signal when playing the multimedia file from the USB port or streaming signal, and can’t hear the sound from the rear speaker, adjust the following settings:
a) Select Settings in the Player menu
b) Select Audio
c) Select Digital audio output (passthrough)
d) Press the OK, Confirm button on your remote control to highlight the control box and confirm your selection.
e) Perform a power reset of the box by unplugging the power supply cable of your box, and few seconds later, plug the power supply cable into the box again.
f) The box will be restarted
g) The power reset will activate the change of the audio output properties.
h) Play the desired multimedia file via the same Application again and set the desired audio settings on your AV amplifier. In this way, the audio signal will be sent to your amplifier unchanged and can be adjusted by you on the AV amplifier individually.
Have you done any of these things?