Since it’s a Victrola, you may not have that 3.5 mm line in connected. You may want to check a few things. First, is the errant speaker plugged in properly? Are the two speakers set up as a stereo pair (in the same software ‘room’) or grouped (two Sonos ‘rooms’)?
You may want to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.