Yes, you can use a non-Roku soundbar on your Roku TV. If you want to connect your Roku TV to an audio/video receiver or soundbar, use the HDMI port that supports ARC (audio return channel). If your AVR or soundbar does not support ARC, you can connect your Roku TV using the optical output.
Note: To use ARC, your AVR or soundbar must support ARC and it must be enabled on that device. Consult your AVR or soundbar product documentation for more information about this requirement.