Sounds like you're using Sonos to play your personal library. This is one of the problems with doing what. Raw levels in music are all over the place. Even the built-in solutions within playback software like iTunes only party solves the problem.
But your real problem is that you ware playing your library for a commercial purpose without a license to do so. It's actually a copyright violation to do what you're doing, an there is an enforcement organization. The fines are quite high.
If you subscribe to a retail music service the license and play levels are taken care of, as well as the music mix that (theoretically) best serves your business and ultimately increases sales. The services can be affordable, there's a wide range of companies and delivery methods (no Sonos, though), and some will even set up your sound system for you.
Google retail music service.
Unless you've paid attention to the system design, it's likely you've used consumer speakers, and probably too few of them. Restaurants that do this end up with sound hot spots where its all way too loud, and quite spots where you can't hear it. Or more likely its too loud in most areas and ear-splitting by the speakers. A professional and can solve this too. It's not all that expensive, and if you create a pleasant and appealing environment you'll have happy customers that repeat, just what you need.