Contact Sonos support. 
 

You might find some test tones online to play and listen for lack-of-sound but the support route will be more definitive. Or send it back to seller as defective -assuming that’s the one that sounds defective. 

Did you run Trueplay on the original one but not the new one?

But perhaps more likely the previous owner blew the mids…

Turn off Trueplay for both.

Set all tone / loudness settings to the same settings.

Group them so they are playing the same stream, then try playing them again and see how they sound.

If still different steal a cardboard tube from a roll of paper towels, use it over your ear with the other end over each driver on the Sonos. If all are playing at least you don’t have a dead one.

Then I’d call support but the wait will likely be long these trying times.

Thanks for the replies everyone.

I'm sure the mids were gone so I've taken the speaker back.