connect your Roku device directly to your TV instead of any Audio/Video Receiver, then you have to do the subsequent steps. First, check whether your Remote is working properly and set the volume high and check the mute settings on your television.
If you are using any composite cable for connecting your Roku stick, check whether the audio connectors are attached properly at the ends.
You can even try with connecting your Roku device to a different input and check whether you can hear the audio. If not, then there is a problem with your input source. Change the cable and try the same process again.
Reference Link: https://www.slideshare.net/MarkAlbert05/roku-no-sound-no-audio-on-roku-roku-customer-support