Whether or not the “mid woofers” are working or not depends upon the music source. Meaning the mid-woofers may be working but the source audio may have very little low end. Try sending the speakers some base heavy audio. 

If you have an iOS device run TruePlay to see if that corrects the issue. If nothing works to your satisfaction I suggest you run a diagnostic within 10 minutes of the occurrence. Post the reference ID in your next post. Then call Sonos tech support. 

Curious as to what ‘woofers’ there actually are in a Sonos One, it’s a fairly small speaker. 

OK I'll try as you say. Sorry if I got the terminology wrong on the speakers, I have no idea. I have tried various songs and the sound is definitely only coming from the smallest speakers at all times. 

There’s some OK images on this website of the internals.

https://blog.bolt.io/sonos-one/

If the larger of the two isn’t working, I think @AjTrek1 has a great suggestion in contacting Sonos support with a diagnostic. 

Thanks for that. It's the larger of the two not working. Considering it's on two speakers at the same time I'm kinda guessing it's gotta be software, which would be good because they're well out of warranty. 

It would strike me as odd for it to be software, since there aren’t thousands of posts complaining about it, and all of mine seem to be working fine. But if it’s a hardware or software issue, that diagnostic that @AjTrek1 mentioned should help Sonos figure it out.