When used as surrounds, the line-in functionality on the Era 300s is disabled.

What about the Bluetooth?

Would it not be availed as a source?

Bluetooth functionality is also disabled when the Era 300s are being used as surrounds. If you want to connect a turntable, you need to add another Sonos device with line-in like a Five, Port, Era 100, or another Era 300.

Is there a plan in the roadmap to make this possible? Seems to be a big miss and it was a disappointing surprise to only learn about this limitation after purchasing a new surround system and trying to connect my turntable

Although Sonos doesn’t comment on roadmaps, I think it fairly unlikely, due to the nature of the connection between a surround speaker and its parent, which is wildly different than the connection between two ‘peer’ Sonos devices.

While there are quire a few turntable fans, unfortunately, in terms of the overall market it’s a miniscule fraction. Many users of regular A/V equipment challenge the inclusion of a PHONO input that they’ll never use because it is a “complication” and it will increase the cost of the unit (slightly).