Sonos doesn’t do support for Victrola, you would need to contact them.

The audio you are hearing through the headphones are coming directly from the turntable and NOT the Sonos speaker. Make sure the turntable has the latest firmware. If it does, you should contact Victrola support.

In addition to the above there is no such product as a “Victrola Sonos Stream”. Victrola advertises their product to “work with Sonos”. Sonos has no direct partnership with Victrola…manufacture, marketing or otherwise.

Also why would you purchase a “pre-amp” unless you can disable the Victrola’s internal pre-amp? However , doing so requires running RCA cables between the Victrola and the Pre-amp and then an RCA to 3.5mm from the pre-amp to the Sonos proprietary line-in adapter to the Era 300. Doing all the above defeats the purpose of purchasing the Victrola for its wireless capabilities.

Do the headphones work OK on other sources?

And on the other side… does BT Audio from the turntable sound well on other BT devices like e. g. the Era 300 speaker?

Yes - headsets are fine on all other devices. And the app says that the software is up to date. And the record player sounds awesome on my e-300 speaker. To AJ Trek1, it was a solution that I found on Reddit.

Ah Ha! The solution on Reddit was probably suggesting a Pre-Amp with a Headphone jack sometimes called a headphone amplifier. Essentially you capture the audio before it goes to your Sonos speaker. My Cambridge Alva Duo pre-amp has a headphone option. However I doubt you’ll find one on Amazon of any brand for $70, then again you may 😊. Good luck and enjoy your Sonos.