Well, supporting ARC is one thing your speakers don't. I just priced soundbars a week ago for my daughter's TV, as they had a couple of speakers die and can't afford a complete 5.1 system at this time. I got them a pretty decent 2.1 soundbar for $150, and that included a sub. It doesn't support ARC, and of course neither it nor your speakers also contain a Roku Ultra within it. So, figuring about $80 of the price is the Ultra, the another $70 for the soundbar is actually a pretty good price. It's adding the sub for an addition $180 is where it's past the bargain level. I'd say a total of about $200, and you get a Roku Ultra, the soundbar and the sub, that would be an exceptional deal.
The Onn. Roku Smart Soundbar soundbar connects to your TV via either HDMI or optical audio (S/PDIF). For more information, we'd recommend contacting Onn support.
You can also see more about the Onn soundbar on Walmart's website here: https://www.walmart.com/ip/Onn-Roku-Smart-Soundbar/642573197