Couple of things I’d check.
First, make sure ARC is actually turned on (I think Sony calls is BraviaSync) so that the Sonos Arc is able to ‘tell’ the TV what kind of codecs it can accept.
Next, check the shows you’re watching. If they are in stereo, then the Sonos will receive stereo, Sonos doesn’t have the ability to change everything to Atmos, or Dolby Digital. Nor do they change the volume of the show, if the engineer has encoded it at a low volume. And because stereo is only using two speakers, not more, it often sounds ‘quieter’.