Are you referring to listening to a streaming service while the TV is on? If so, simply go into the app and select the service as the source. You can always change it back to TV audio later.

If you are referring to  increasing or lowering the volume of the Arc separately, using the app instead of the TV remote does this.

EDIT: If the TV speakers are playing the same time as the Arc, you must go into your sound settings and select the output device found here:

https://www.lg.com/us/support/help-library/lg-tv-how-to-connect-a-sound-bar-or-receiver--20153207557627

Scroll to the “Connect using HDMI cable” section.

Surely you want to listen to the sound just from the Arc, rather than with tv speakers playing as well? It must sound pretty muddled and echoey through both? Disable the tv speakers. 

yes, I want to listen to the sound just from the Arc.  many thanks for both answers