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Yes, you could add a soundbar. You would connect it to the TV either with an optical cable, or if both support ARC, via an HDMI cable. You say the TV doesn't have audio outputs, but I'd be surprised if there wasn't an optical connection available.
If you bought one of the Roku soundbars, you'd have a Roku player built into it. Roku also sells some wireless speakers, but they will only connect to a Roku TV, not a Samsung. You can also use them as rear surround speakers if you connect them to a Roku soundbar.
But it doesn't have to be a Roku soundbar. You could continue using your existing Roku and just add another brand of soundbar to the TV. Some soundbars either come with or support adding a subwoofer, and the combination would sound vastly superior to the TV speakers.
Finally, you could go all in and add an AV Receiver with 5 speakers plus a subwoofer. But that's going to be far more expensive, even for bargain level products.
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