At the time you attempt this, are you watching a Dolby Digital (minimum) signal? Speech enhancement wouldn’t ’turn on’ on a stereo signal. 

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

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.

@jorzzwy13 as you use an AppleTV it has speech enhancement in the OS which to my ears works better than the Sonos implementation but YMMV.  There’s also a night mode which works great too  

https://support.apple.com/en-ca/guide/tv/atvba773c3c9/tvos

Are you trying to enable Speech Enhancement from the icon TV Now Playing screen or from the menu under Room settings?

If the former, then try the latter.

If that also fails, if you force close and reopen the app, does the icon then work?

The Sonos app is buggy. Keep restarting the app until Speech Enhancement works again.