You're going about the connections wrong. According to an online manual I've seen for your soundbar, it has both an HDMI IN
and an HDMI-ARC OUT. You need to connect the Roku Premiere+ directly to
the soundbar's HDMI-IN port. Leave the soundbar's HDMI-ARC OUT port connected like it is now, directly to the LG TV's HDMI-ARC IN.
Set the Roku's audio for "Auto".
Once set, reboot the Roku by unplugging it and plugging it back in again. (Note: To watch the Roku, you will no longer be using the TV's HDMI-3 source. You'll be using whatever the LG TV's HDMI-ARC input source is.)
See "
Option 1" in this Vizio PDF:
Vizio 36512-F6 Soundbar PDF (In case that's not clickable, here's the URL: http://cdn.vizio.com/user-manual/PDF/2018/SB/SB36512-F6_QSG_ENG.pdf)
Basically, all you are doing is removing the the end of the Roku HDMI cable you've got connected to the LG's HDMI-3 and instead connecting it to the soundbar's HDMI-ARC IN input. So you'll be connecting the Roku directly to the HDMI-IN on your soundbar, vs HDMI-3 on your TV.