New to Sonos Playbar world, how to get ROKU to do 5.1 sound

That Monoprice one has been suggested a number of times on the community and I've seen it work well. And yes, that HDMI out from the TV to the switch should do it, unless all your sources are HDMI, and then you can just skip the TV and have them all go to the switch instead.

I use my TV antenna, and a ROKU, thats it.... somewhere I read something about ARC, it looks like the standard monoprice doesn't have it, will it work?

The HDMI switch does need to support ARC in order to get the TV audio and then pass it along to the audio component (such as a PLAYBAR or PLAYBASE). You may need to look at an HDMI switch such as this one here. However, keep in mind that not all HDMI switches process audio at the same rate and that this switch allows for the optical output to be turned on and off (so an accidental button press can leave you without audio).

Given your setup, I'd probably opt for the optical switch over the HDMI one. I've used the Monoprice 5557 before and it was fine, but unnecessary if you only have one HDMI source and cleaner to not have to mess with ARC. Go for one that does not have auto-switching (meaning that it switches to the active input) or at least has an option to disable it as your TV will always be outputting over optical in this setup and it could make it tricky to get it to switch to the Roku.

Couple other important considerations here.

I'm assuming your Roku actually has an optical output as that's what you're proposing to use. Most of them don't, however, so confirm that if you haven't already - I believe it's only the last-gen Roku 4 and Ultra that do along with maybe some very early models.

2nd issue is that many streaming sources encode in Dolby Digital Plus rather than plain Dolby Digital. DD+ cannot travel over optical, so unless the device can do the conversion, you'll only be able to get stereo, defeating the purpose of this more-complicated setup. Assuming again that you have one of the few models with an optical out, you'll want to confirm that the box will do this conversion (I know the Roku Ultra has a setting to do so, but not sure about others). If the device won't convert DD+ to DD, there's no way to get 5.1 to the Playbar.

Only other scenario I can think of is if if you don't have an optical out on the Roku, but the Roku will do DD+ to DD conversion. Latest generation of boxes don't have the optical, but I'm not sure if they will do the conversion (I suspect not). If this is the case, though, you'll want one of those HDMI switches with optical out. I don't have any experience with ARC, so can't comment on how reliably you'd be able to get audio from your TV to go through that switch to the Playbar. One option would be the HDMI switch and the optical switch working together, but that's probably more complicated than you want.

Going to correct myself here as I think the Roku Ultra is actually a current-generation model and appears to be the only one that has an optical out and the ability to transcode DD+ to DD. Seems to me that unless you have this model or an older one with similar features (if that exists), you're out of luck. Might be worth the $100 and change to get it, though, if it's the primary way that you use your TV.

I have the same problem with my new Roku Ultra. When streaming Netflix via my Roku Ultra (2017), my Samsung UN60KS8000F TV is unable to pass through a DD 5.1 signal to my Sonos Playbar. My Roku Ultra is connected to my Samsung via an HDMI cable, and my Sonos Playbar is connected to my TV via an optical cable. My Play Bar is also set up for 5.1 (Connect Amp and a Sub). My Sonos app shows a my Playbar audio in as "stereo".

When I stream Netflix via my TV app I am able to stream movies and hear DD 5.1, however, when I stream Netflix via my Roku Ultra, my Samsung does not recognize DD 5.1. When I stream Netflix via my Roku Ultra, my Samsung audio format only shows PCM and DTS Neo 2:5, The Dolby Digital and DTS is greyed out. Just wanted to know why this is happening, it is more convenient to use my Roku Ultra for watching live TV and Netflix, thanks in advance for any suggestions.