Try setting the Source Level higher under the Line-In settings for the Era 100 in the Sonos app.

Thank you! The volume is fine though, it’s just not the same quality and depth of sound of the same song in spotify.

I’ve not been able to find any English language documentation. A partial translation indicates that the turntable does offer a built-in preamp. Make sure that you are using the correct set of output jacks on the turntable and, if there is a switch setting on the turntable that it is set to preamp output. If you are skipping the preamp the turntable output will be very low and there will be excessive treble.

If the tracking force is misadjusted the sound could be distorted.

You mention the sound is close to the built in speakers but worse than streaming. A bit of that is to be expected because the record has lower fidelity than the digital stream

You could try improving the sound for both the speakers and Sonos by cleaning your stylus, checking for wear/damage and then checking alignment and tracking force. All a bit complicated to do and beyond the Sonos forum focus but any good turntable forum should get you good instructions.

When all is proper it should sound pretty good over the Sonos and speakers.