The reason for the sound jam of optical fiber output decoder may be that the network speed is slow due to network reasons, or it may be that the decoder has no analog audio output and only a digital coaxial output interface.

You can update the network speed or buy a decoder with coaxial digital and optical fiber to analog av output for normal connection.

Decoder is a hardware/software device that decodes digital video-audio data stream and restores it to analog video-audio signal.