Likely as Sonos has yet to release a ‘fix’ to the current Bluetooth issue. 
 

Odd that the connection on the back of the turntable is a male RCA jack, that is counter to the industry standard, but there are certainly female RCA to male 3.5mm cables out there, although I suspect they’re a lot more rare, given the fact that the market for them must be tiny. I’d look on Amazon, given the rarity, it’s unlikely that a Best Buy would ever carry them. Unlikely to be there in time for a Christmas gift, though. 
 

Just a quick reminder, the line in on an Era 100 is turned off when it is in use as a surround speaker. 

You’d not need just “a 3.5mm to usb-c adapter”, but you need the specific Sonos adapter. You should be able to connect that to the Sony via a cable like this: https://www.walmart.com/ip/Stereo-Male-Jack-To-2-RCA-Female-Plug-Adapter-Headphone-3-5mm-T8S7/2292689110?athbdg=L1700

I’m at my wits end. This stuff costs entirely way too much to have this much trouble getting it to simply play sound. I can’t find a a female rca to 3.5 adapter anywhere, nor does anyone have the Sonos 3.5 to usbc. So I broke down and got the port. It’s all set up, connected to wifi, the male rca from the turntable are plugged directly into the back of the port. Everything is linked in the app as far as I can tell. The turntable is set to line not Bluetooth. Still no sound. What am I missing here?! 

Did you choose the Line-In as a source?  Or set the Autoplay settings to play to the room you want to play when the Port detects a Line-In signal.  

https://support.sonos.com/en-us/article/listen-to-your-turntable-on-sonos

I think I finally figured it out. Auto play was on the wrong “room”. The port and the speakers show up on different rooms for some reason. Everything was selected to be on “living room” when each device was set up, but it automatically made a “living room 2” for the port device while the era 100s are on the original “living room.” Is that how it should be? 

Yeah.  I'd rename the Port to "Port" if you are going to use the outputs.  Less confusing that way.

New issue, while I got the port I decided to try a Five maybe instead of the era 100s. Powered on and it’s not asking if I want to add my new device like it has for everything else. 

You need to initiate by choosing "Add a new product" in the app.  The Five predates the BLE sensing of the newer units.

It wouldn’t find it that way either. Turns out it was an open box sale and the memory had to be wiped. It’s now connected. 
 

one last thing I’ve noticed. The volume is FAR LOUDER when streaming from my phone vs the turntable. Any ideas? It’s probably 2-3 times the volume. 

You can adjust the Line-In level in the settings.  Increase it until it matches the streaming level.

Everything is hooked up and working great. Sounds amazing streaming from my phone, wouldn’t change a thing. The line in is still lacking a bit. I did find the line in level adjustments in the app. I set it all the way up on level 10. It helped a lot, however, it’s still nowhere close to the levels when streaming from my phone. Probably still 25%+ less volume and bass. Is this just an attribute of vinyl? 

It may be the case that your pre-amp isn't engaged or may be missing.  If your turntable has a built-in preamp, make sure it is set to Line output.  If it doesn't have a preamp, you need to get an external phono preamp to wire between the turntable and the Era 100.

PS - That turntable does have a preamp.  On the back of the unit, the switch labeled "Output" should be set to "Line".

It has one built in and didn’t work until I switched it to line out so I assume it’s working. The volume is enough to fill an approx 700sq ft room as long as there’s not a lot of other ambient noise. I’ve Just noticed I’m around 30-40% on the volume control with streaming vs 75% for line in. 

Great.  You can also increase output by setting the "Gain" switch to High.

That made a big difference. Thanks .