ARC setting on your Television vary from manufacturer to manufacturer. So please double check your TV setup manual.
Common issues for the lack of ARC audio: 1. The standard Linn DSM can only playback stereo audio. If the TV is set to output Dolby Digital or other Surround format then the Linn DSM will not play this audio and stay silent. make sure the audio output is set to "STEREO"'
2. The TV audio setup may be conflicting with itself. If possible do an "AV RESET" or equivalent on your TV, then setup for Audio format - Stereo and CEC-enabled (CEC can go by different names AnyNet+, Veiralink, SimpLink, RIHD, Kuro Link..... etc. Check what your TV call it)
3. You are connect to the wrong socket on your TV. Make sure it states "ARC"