I'm not sure what you are asking to do.
If you just don't want any sound, use the mute button on your TV.
But, if you want the video on your TV and the sound on your device (computer, phone, tablet, etc) you can't do that. Casting sends the audio/video together and you can't split the stream and keep the audio local.
We haven't heard back from MartinG, and it's been a few days, so I'm locking the thread. As always, feel free to make a new thread if you have any more questions or concerns.
Thank you for your help, Byteguy!