If the Beam is outside of the warranty or any extended warranty period, you may find repairers online, Another community user posted this link (just as one such example) recently, but obviously he/I cannot verify if the chosen repairer will be up-to-the-mark, so such a course of using 3rd-party repairers is entirely at your own risk
Just as an example:
https://www.isonatec.com
I can’t personally comment on the audio that you’re hearing from your Beam, as connected audio sources can and do vary and environments vary, aswell as each persons hearing.
If you perhaps think the Beam is faulty in some way however, before doing anything else I would submit a Sonos system diagnostic report and post it’s reference back here.. then follow this support LINK to contact/chat to Sonos Support Staff about the matter, as the diagnostic report may reveal if there is a problem with your speaker.
I’d also check your system to ensure the signal reaching the Beam is indeed Dolby Digital (5.1), and not just a stereo or PCM 2.0 signal, which could potentially cause dilution of the center channel, as the center channel wouldn’t then exist. It’s possible that the settings on a source device feeding your TV and the Sonos has been changed without your knowledge on a ‘background’ update.
Deleted: already suggested above.