It depends on the source, it could be that when you’re placing your ear against the PLAYBAR, the only thing being exercised by the source is the center channel.
Try streaming some music, which will play on the left and right set of speakers, and not the center, to check if those other speakers are working. And contact Sonos support, they’d likely have other suggestions.
Oh, you may want to double check both the audio settings on the TV, in addition to any device connected to the TV as a source, and make sure they’re set to send a Dolby Digital 5.1 signal, when one is available. Since optical is a one way transmission, there’s no opportunity for Sonos to use CEC.