Either transfer it to a laptop/PC/NAS of some type, and set it up as your Sonos Library using the instructions in the Sonos FAQ, or if you have AirPlay 2 capable Sonos speakers, use AirPlay 2 to connect to your system, and use the Music app on your iPhone to send the music to the Sonos speaker. 

What Bruce said. Or, your profile says you have an Era so you could Bluetooth-pair phone to Era. 

Where does the music for your phone come from? Do you have a Mac with a local library that syncs to your phone?

I’m trying to figure out the same thing.  I was able to do a work around by mirroring my screen to my TV and have it play through there.   I hope there is a better way.  You shouldn’t have to subscribe to Apple Music just to listen to your onboard library through Sonos.

Apple is the reason why you can’t play music stored on your phone directly using Sonos. They blocked the ability that Sonos used to use in the past to access that music. Which is why we have to suggest lots of random ways, from AirPlay 2 or Bluetooth when available, to placing the music on another device.