Try the Remote Control Setup under the Arc settings in the Sonos app:

https://support.sonos.com/s/article/3402?language=en_US

Does the Samsung Solar Cell Remote use a “smart” connection, or does it use IR? Since you’re connected via optical, the Arc will only respond to IR signals.

Optical is a one way signal, and CEC commands such as volume are not carried backwards by that connection.

You may find some applicable help in the  Samsung One Remote FAQ , although it depends on how that particular Solar Cell Remote works...

You want to setup the SAMSUNG remote to control an arbitrary device via IR. Here are some SAMSUNG instructions. I don’t have one of these sets in front of me, but basically you want to control Volume on an arbitrary device (pick a device that is not nearby). After you’ve trained the SAMSUNG remote, then train ARC to respond to these IR commands. It is usually most convenient to train both your cable box remote and the TV remote to control the same arbitrary device.