Is this only one ERA 300?  Do you mean the front facing speaker is not working?

Contact Sonos support and have them check it out. 

Playing any music to the 300 should get sound from that speaker. Same if it is part of a stereo pair.

If it is being used as a surround that speaker is disabled. Much discussion of that here already if you want to know more.

Only one speaker

Yes front facing speaker 

I am requesting community to tell some tips to check center speaker or front facing speaker is working or not? I couldn’t feel sound is coming from center speaker ( I bought speaker last month it’s very new )
 I have some more information to give  solution for my question

• Dolby atmos enabled songs playing via Sonos app
• you tube center speaker test was also done 
• no internet issues
• solid white light on speaker
• speaker featured with Dolby atmos but no feeling it’s atmos  speaker 

 When you put your ear up against the front of the 300 do you hear anything?  My 300s are bonded as surrounds and when I put my ear against the front of the speaker there is no sound which is the way it’s supposed to be.

 When the 300 is playing as a single speaker or two 300s as a stereo pair there should be sound coming from the front firing speaker.  The fact there are side and top firing speakers in the 300 the sound will not be as specific or direct as a traditional speaker.  I find this to be advantageous.

 An ideal well set up stereo system should not make it obvious there is a left and right speaker unless the source of the music is poorly mastered.  The speakers should just disappear, at least when your eyes are closed.