The Onn Roku soundbar remote is IR, and its power & volume commands are not in any preprogrammed remote. These commands are Roku commands, they can't be set to different tv brands. These commands simply tell the soundbar to send a hdmi-cec signal to the tv its connected to, and ALL tv brands respond to the same cec-signal for tv power, volume up & down, and mute. So the soundbar remote controls the tv by hdmi-cec, where your cable remote controls the tv by IR.
You have Roku tv and Roku soundbar, the only cable you need is a hdmi cable connected to the ARC input on the Roku tv, probably Hdmi 3. Never use the ARC and optical at the same time. The optical cable is only there if your tv didn't have an ARC input, which it does, so discard the optical cable.
I've done everything and still no sound
Refresh the cec devices in the Roku tv settings:
settings> system> control other devices> search cec devices> ok
